Crude and adjusted comparisons of cesarean delivery rates using the Robson classification: A population-based cohort study in Canada and Sweden, 2004 to 2016.

BACKGROUND: The Robson classification has become a global standard for comparing and monitoring cesarean delivery (CD) rates across populations and over time; however, this classification does not account for differences in important maternal, fetal, and obstetric practice factors known to impact CD rates. The objectives of our study were to identify subgroups of women contributing to differences in the CD rate in Sweden and British Columbia (BC), Canada using the Robson classification and to estimate the contribution of maternal, fetal/infant, and obstetric practice factors to differences in CD rates between countries and over time. METHODS AND
FINDINGS: We conducted a population-based cohort study of deliveries in Sweden (January 1, 2004 to December 31, 2016; n = 1,392,779) and BC (March 1, 2004 to April 31, 2017; n = 559,205). Deliveries were stratified into Robson categories and the CD rate, relative size of each group and its contribution to the overall CD rate were compared between the Swedish and the Canadian cohorts. Poisson and log-binomial regression were used to assess the contribution of maternal, fetal, and obstetric practice factors to spatiotemporal differences in Robson group-specific CD rates between Sweden and BC. Nulliparous women comprised 44.8% of the study population, while women of advanced maternal age (≥35 years) and women with overweight/obesity (≥25 kg/m2) constituted 23.5% and 32.4% of the study population, respectively. The CD rate in Sweden was stable at approximately 17.0% from 2004 to 2016 (p for trend = 0.10), while the CD rate increased in BC from 29.4% to 33.9% (p for trend < 0.001). Differences in CD rates between Sweden and BC varied by Robson group, for example, in Group 1 (nullipara with a term, single, cephalic fetus with spontaneous labor), the CD rate was 8.1% in Sweden and 20.4% in BC (rate ratio [RR] for BC versus Sweden = 2.52, 95% confidence interval [CI] 2.49 to 2.56, p < 0.001) and in Group 2 (nullipara, single, cephalic fetus, term gestation with induction of labor or prelabor CD), the rate of CD was 37.3% in Sweden and 45.9% in BC (RR = 1.23, 95% CI 1.22 to 1.25, p < 0.001). The effect of adjustment for maternal characteristics (e.g., age, body mass index), maternal comorbidity (e.g., preeclampsia), fetal characteristics (e.g., head position), and obstetric practice factors (e.g., epidural) ranged from no effect (e.g., among breech deliveries; Groups 6 and 7) to explaining up to 5.2% of the absolute difference in the CD rate (Group 2: adjusted CD rate in BC 40.7%, adjusted RR = 1.09, 95% CI 1.08 to 1.12, p < 0.001). Adjustment also explained a substantial fraction of the temporal change in CD rates among some Robson groups in BC. Limitations of the study include a lack of information on intrapartum details, such as labor duration as well as maternal and perinatal outcomes associated with the observed differences in CD rates.
CONCLUSIONS: In this study, we found that several factors not included in the Robson classification explain a significant proportion of the spatiotemporal difference in CD rates in some Robson groups. These findings suggest that incorporating these factors into explanatory models using the Robson classification may be useful for ensuring that public health initiatives regarding CD rates are evidence informed.

Introduction

In 2015, the World Health Organization (WHO) endorsed the Robson classification system as a global standard for comparing and monitoring cesarean delivery (CD) rates across populations and over time [1]. As a result, the use of the Robson classification in evaluating CD trends has expanded over the last decade and become a common tool to inform obstetric practice worldwide [2-7]. This classification scheme stratifies deliveries into 10 mutually exclusive and all-inclusive categories based on 6 obstetric characteristics: parity (nulliparous versus parous), previous CD (yes/no), plurality (single versus multiple fetuses), fetal presentation (cephalic, breech, transverse/oblique), labor onset (spontaneous, induced, prelabor CD), and gestational age (<37 versus ≥37 weeks) [8,9].

The Robson classification system represents an elegant method that uses stratification (to control for confounding) in order to isolate the effect of specific obstetric practices on CD rates. Nevertheless, there are important determinants of spatial and temporal variations in CD rates that are not integrated into the Robson analysis scheme [10,11]. For example, maternal characteristics such as age [12] and body mass index (BMI) [13] are 2 important determinants of CD rates that are not addressed by the Robson strata. Similarly, obstetric practice factors, such as the clinical management of postterm delivery [14], as well as fetal factors, such as position of the fetal head at delivery [15] and fetal size [16], have also been associated with CD but are not considered in the Robson scheme. Inferences about spatiotemporal differences in CD rates may be biased if they are based on patterns within the Robson groups but without appropriate consideration of these extraneous factors that influence CD rates [5].

Statistical adjustment using regression techniques can address the contribution of extraneous determinants (such as maternal age and BMI) so that the true association between obstetric practice and CD rate can be quantified. The few studies that have applied regression to the assessment of CD rates using the Robson framework have been limited by single-center or cross-sectional study designs [17,18], while prior population-based or longitudinal studies lacked information on important risk factors for CD (e.g., labor onset, early-pregnancy BMI, maternal comorbidity, fetal factors) [5-7,19-21]. As a result, the effect of maternal, fetal, and obstetric practice factors (not represented in the Robson classification) on international and temporal comparisons of CD rates using the Robson classification is unknown.

We carried out a study to identify subpopulations responsible for differences in CD rates between Sweden and British Columbia (BC), Canada using the Robson classification system. Further, we sought to quantify the contribution of maternal characteristics, obstetric practice factors, and fetal characteristics to variations in the CD rate (a) between countries; and (b) within each country over a 13-year period.

Methods

This study is reported as per the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline (S1 STROBE Checklist). The study methodology and analysis plan (S1 Text) were developed prior to data extraction and the actual analysis.

We conducted a population-based, retrospective, cohort study including all deliveries (live births and stillbirths) ≥22 completed weeks of gestation in Sweden and BC. Data for the Swedish cohort of deliveries between January 1, 2004 and December 31, 2016 were obtained from the Swedish Medical Birth Register (MBR), a validated, nationwide health register containing more than 98% of all births in Sweden [22]. Data for the Canadian cohort were obtained from the British Columbia Perinatal Database Registry (BCPDR) for the fiscal years 2004/2005 to 2016/2017 (hereafter referred to as years 2004 to 2016). The BCPDR contains information on approximately 99% of births in the province of BC [23]. Validation studies show that the BCPDR is an accurate and comprehensive source of perinatal information [24,25].

Both the MBR and the BCPDR contain detailed demographic and clinical information on all mothers and babies including information on diagnoses and procedures classified using standardized codes [22,23]. The study was restricted to the years 2004 to 2016 when all diagnostic codes conformed to the 10th revision of the International Statistical Classification of Diseases and Related Health Problems in Sweden (ICD-10-SE) [26] and Canada (ICD-10-CA) [27]. Similarly, procedure codes were consistently coded over the study period with the Nordic Classification of Care Measures [28] in Sweden and the Canadian Classification of Health Interventions in Canada [29].

Analysis of CD rates using the Robson classification system

Data were grouped into Robson categories and modified from the 10-group system to a 12-group system, used commonly [9] to differentiate women in Groups 2 and 4 between those who had an intrapartum CD after induction (defined as 2a and 4a) and those who had a CD before labor onset (2b and 4b; Table 1). Women with twin (or higher order) pregnancies were only counted once; the mode of delivery of the second twin was used in the analysis. The overall CD rate, the relative size of each Robson group, and the absolute contribution of each group to the overall CD rate were compared between the Canadian and Swedish cohorts using the Standard Robson Classification Report Table. We also assessed the data quality, population distributions, and Robson group-specific CD rates against WHO-recommended benchmarks using standardized criteria [1]. Temporal trends in CD rates were described by Robson group and assessed using the Cochran–Armitage test. Temporal trends in the relative contribution of each Robson group to the overall CD rate were also described.

Table 1

Distribution of maternal, obstetric practice, and fetal/infant characteristics among deliveries in Sweden and BC, Canada, 2004–2016.

Maternal, obstetric practice, and fetal/infant characteristic	All deliveries (N = 1,951,984)	Sweden (n = 1,392,779) No. (%)	BC (n = 559,205) No. (%)	Standardized difference*
Maternal age (year)				0.17
<20	29,166 (1.5)	19,859 (1.4)	9,307 (1.7)
20–24	234,269 (12.0)	175,914 (12.6)	58,355 (10.4)
25–29	548,949 (28.1)	412,828 (29.6)	136,121 (24.3)
30–34	676,342 (34.6)	482,275 (34.6)	194,067 (34.7)
35–39	373,699 (19.1)	247,619 (17.8)	126,080 (22.5)
40–44	84,413 (4.3)	51,616 (3.7)	32,797 (5.9)
≥45	5,146 (0.3)	2,668 (0.2)	2,478 (0.4)
Maternal early-pregnancy BMI (kg/m2)				0.63
Underweight (<18.5)	50,221 (2.6)	28,394 (2.0)	21,827 (3.9)
Normal weight (18.5–24.9)	1,000,635 (51.3)	759,464 (54.5)	241,171 (43.1)
Overweight (25.0–29.9)	419,302 (21.5)	333,738 (24.0)	85,564 (15.3)
Obese class I (30.0–34.9)	144,819 (7.4)	112,532 (8.1)	32,287 (5.8)
Obese class II (35.0–39.9)	48,684 (2.5)	35,785 (2.6)	12,899 (2.3)
Obese class III (≥40.0)	19,946 (1.0)	12,911 (0.9)	7,035 (1.3)
Missing	268,377 (13.7)	109,955 (7.9)	158,422 (28.3)
Parity				0.04
0	875,011 (44.8)	615,451 (44.2)	259,560 (46.4)
1	713,765 (36.6)	512,130 (36.8)	201,635 (36.1)
2	252,053 (12.9)	185,055 (13.3)	66,998 (12.0)
3–4	93,654 (4.8)	67,496 (4.8)	26,158 (4.7)
≥5	17,319 (0.9)	12,645 (0.9)	4,674 (0.8)
Missing	<185 (0.0)	<5 (0.0)	180 (0.0)
Smoking during pregnancy	143,148 (7.3)	95,314 (6.8)	47,834 (8.6)	0.06
Preexisting diabetes	9,660 (0.5)	6,715 (0.5)	2,945 (0.5)	0.01
Preeclampsia/eclampsia	47,990 (2.5)	39,509 (2.8)	8,481 (1.5)	−0.09
Chronic hypertension	13,754 (0.7)	10,000 (0.7)	3,754 (0.7)	−0.01
In vitro fertilization	50,015 (2.6)	39,590 (2.8)	10,425 (1.9)	−0.07
Onset of labor				0.28
Spontaneous	1,410,549 (72.3)	1,049,748 (75.4)	360,801 (64.5)
Induced	331,153 (17.0)	213,499 (15.3)	117,654 (21.0)
CD before labor	202,714 (10.4)	121,978 (8.8)	80,736 (14.4)
Unknown	7,568 (0.4)	7,554 (0.5)	14 (0.0)
Gestational age (completed weeks)				0.29
Very early preterm (22–27)	7,062 (0.4)	4,663 (0.3)	2,399 (0.4)
Early preterm (28–31)	11,677 (0.6)	7,695 (0.6)	3,982 (0.7)
Late preterm (32–36)	107,720 (5.5)	63,490 (4.6)	44,230 (7.9)
Term (37–41)	1,723,126 (88.3)	1,221,617 (87.7)	501,509 (89.7)
Postterm (≥42)	101,377 (5.2)	94,880 (6.8)	6,497 (1.2)
Missing	1,022 (0.1)	434 (0.0)	588 (0.1)
Epidural anesthesia	601,490 (30.8)	426,192 (30.6)	175,298 (31.3)	0.02
Vacuum	137,204 (7.0)	98,043 (7.0)	39,161 (7.0)	0.00
Forceps	19,547 (1.0)	2,444 (0.2)	17,103 (3.1)	0.23
Plurality				0.08
Singleton	1,923,311 (98.5)	1,373,078 (98.6)	550,233 (98.4)
Multiple	28,673 (1.5)	19,701 (1.4)	8,972 (1.6)
Infant birth weight (g)				0.20
<2,500	80,830 (4.1)	52,562 (3.8)	28,268 (5.1)
2,500–2,999	231,533 (11.9)	148,620 (10.7)	82,913 (14.8)
3,000–3,499	651,997 (33.4)	445,767 (32.0)	206,230 (36.9)
3,500–3,999	657,834 (33.7)	484,302 (34.8)	173,532 (31.0)
4,000–4,499	266,752 (13.7)	209,206 (15.0)	57,546 (10.3)
≥4,500	60,728 (3.1)	50,385 (3.6)	10,343 (1.8)
Missing	2,310 (0.1)	1,937 (0.1)	373 (0.1)
Infant head circumference at birth (cm)				0.16
<33	120,738 (6.2)	77,129 (5.5)	43,609 (7.8)
33–34	610,852 (31.3)	418,513 (30.0)	192,339 (34.4)
35–36	913,013 (46.8)	661,093 (47.5)	251,920 (45.0)
≥37	270,225 (13.8)	204,664 (14.7)	65,561 (11.7)
Missing	37,156 (1.9)	31,380 (2.3)	5,776 (1.0)
Fetal head in occiput posterior position at delivery	92,042 (4.7)	59,495 (4.3)	32,547 (5.8)	0.07
Congenital anomaly	77,042 (3.9)	48,686 (3.5)	28,356 (5.1)	0.08

*Standardized difference values >0.1 are considered indicative of a significant difference between groups.

Small numbers (<5) were suppressed in order to prevent potential identification and breach of confidentiality (cells were sometimes suppressed to prevent back calculation).

BC, British Columbia; BMI, body mass index; CD, cesarean delivery; No., number.

Contribution of maternal, fetal, and obstetric practice factors to spatial differences in CD

We categorized potential determinants of CD into 4 groups: maternal characteristics, maternal comorbidity, factors related to obstetric practice, and fetal factors. The maternal characteristics included age (<20, 20 to 24, 25 to 29, 30 to 34, 35 to 39, 40 to 44, ≥45 years), parity (in parous groups; 1, 2, 3 to 4, ≥5), early-pregnancy BMI classified in kg/m2 as normal (18.5 to 24.9), underweight (<18.5), overweight (25.0 to 29.9), obesity class 1 (30.0 to 34.9), class 2 (35.0 to 39.9), and class 3 (≥40), and maternal smoking during pregnancy. In Sweden and BC, maternal BMI in early pregnancy was calculated from self-reported height and weight or provider assessment at the first antenatal visit, which typically occurs within the first 14 weeks of gestation [22,23]. We also adjusted for maternal comorbidity, which included chronic hypertension, early-pregnancy diabetes, in vitro fertilization, and preeclampsia/eclampsia. Epidural anesthesia and postterm delivery (≥42 completed weeks gestation) were the obstetric practice factors identified as potential determinants of CD. We chose these 2 obstetric practice factors because (a) we hypothesized they would have substantial variability between the 2 countries and over time; and (b) this information was reliably captured in both the MBR and the BCPDR. Finally, we also adjusted for fetal/infant characteristics: position of the fetal head at delivery (in groups with a cephalic-presenting fetus; occiput posterior versus occiput anterior), birth weight, head circumference, and congenital anomaly.

The distribution of these risk factors for CD were compared between Sweden and BC using frequencies and standardized differences [30]. Modified Poisson regression with robust standard errors was used to estimate the effect of maternal characteristics, comorbidity, fetal characteristics, and obstetric practice factors on the differences between CD rates in each Robson group in BC versus Sweden. These analyses were performed on individual pregnancies, which made it possible for a woman to contribute more than 1 pregnancy in the Robson groups that included multiparous women. Therefore, we used generalized estimating equations (with the mother’s identification as a cluster and assuming an exchangeable correlation structure), with adjustment for the possible correlation in outcome that could be introduced by subsequent births by the same mother. The difference between CD rates in the 2 countries that was explained by differences in risk factors was assessed by comparing the crude rate ratio (RR) and adjusted RR (ARR) in each Robson group and on the absolute scale, by comparing the crude and adjusted CD rate in BC versus Sweden. Missing BMI values were modeled using the missing-indicator method. Records with missing values for other variables were excluded as they did not exceed 3% of the total study population; thus, we conducted a complete-case analysis with respect to these variables (Fig 1).

Fig 1

Flow diagram showing derivation of cohort.

We calculated RRs instead of odds ratios (ORs) in our spatial analyses of Robson group-specific CD rates, as the RR is more readily interpretable and the OR does not correspond to the RR when the event rate is relatively high. However, the RR is artefactually constrained when the baseline CD rate is high (e.g., cannot exceed 2 if the baseline CD rate is 55%). The OR is not susceptible to this constraint, and therefore, we also calculated ORs and 95% confidence intervals (CIs) for Robson group-specific CD rates in BC versus Sweden.

Contribution of maternal, fetal, and obstetric practice factors on temporal changes in CD

We quantified the temporal trend in CD rates in Sweden and BC by year and also by period (2004 to 2007, 2008 to 2010, 2011 to 2013, and 2014 to 2016) in the 3 Robson groups with the largest contributions to the overall CD rate. The magnitude of the change in CD rates over time was estimated using RRs and 95% CIs.

To evaluate the contribution of changes in maternal, fetal, and obstetric practice factors to temporal changes in CD rates in these Robson groups, we first compared the temporal trends in the risk factors listed above by period (2014 to 2016 versus 2004 to 2007) using RRs and 95% CIs (the linear trend by year was assessed using the Cochran–Armitage test). Secondly, since changes in maternal characteristics (e.g., increases in advanced maternal age) and maternal comorbidity (e.g., diabetes) can lead to changes in obstetric practice factors (e.g., increases in epidural use), we used sequential log-binomial models to identify the effect of each group of factors on the relationship between year or period and CD. The sequential approach was carried out by fitting a series of models with additional groups of factors added to each model in the sequence outlined above to quantify the contribution of each group of factors to CD trends over time. Temporal trends were estimated both for individual year and period through separate regression models.

Sensitivity analyses

Antenatal and maternity care in Sweden and Canada is universal and free; however, the model of maternity care differs substantially between countries. In Sweden, midwives are the primary care providers for women with low-risk pregnancies [31], whereas in BC, midwives are involved in maternity care for about 25.3% of deliveries [32]. To address differences in maternity care models in each country, our first sensitivity analysis compared deliveries in Sweden with those in BC restricted to women with midwifery care.

We performed 2 additional sensitivity analyses to address missing information about early-pregnancy BMI (13.8% of the study population; 7.9% and 28.3% among deliveries in Sweden and BC, respectively). First, we conducted complete-case regression analysis by excluding deliveries without data on BMI from the regression models. Second, missing values on BMI were estimated using a Markov Chain Monte Carlo multiple imputation method. Results from 10 multiple imputation cycles were combined with the use of PROC MIANALYZE in SAS.

The a priori level of statistical significance was set at a 2-sided p-value < 0.05 for all analyses. The Cochran–Armitage test was used to test for significance of linear trend, and the Wald chi-square test was used to test significance of differences in CD rates in the crude and adjusted regression models. All analyses were conducted using SAS version 9.4 (SAS Institute, Cary, North Carolina, United States of America). The study was approved by the Clinical Research Ethics Board at the University of British Columbia (H14-00674) and the Research Ethics Committee at Karolinska Institutet, Stockholm, Sweden (No. 2008/1182-31/4).

Results

The study population included 1,392,779 deliveries in Sweden and 559,205 deliveries in BC from 2004 to 2016. Nulliparous women comprised 44.8% of the study population, while women of advanced maternal age (≥35 years) and women with overweight or obesity (≥25 kg/m2) constituted 23.5% and 32.4% of the study population, respectively. Inductions occurred in 17.0% of deliveries, while 10.4% were CD before labor. Approximately 6.5% of deliveries occurred at preterm gestation (<37 weeks) and the proportion of deliveries that followed a multifetal pregnancy (twins or higher order) was 1.5% (Table 1). The overall rate of CD was 17.3% in Sweden and 31.2% in BC (Table 2). The rate of CD was higher in BC compared with Sweden in all Robson groups except in Group 4 (parous women with a single, cephalic fetus at term gestation with an induction or CD prior to labor) and among deliveries with a transverse/oblique-presenting fetus (Group 9). The standard assessment of the data quality, obstetric population distribution, and Robson group-specific CD rates is included in S1 Table. The largest differences in the CD rate between BC and Sweden were among women with at least 1 previous CD and a single, cephalic fetus at term gestation (Robson Group 5, OR in BC versus Sweden 4.09, 95% CI 4.00 to 4.18, p < 0.001) and among nulliparous women with a single, cephalic fetus at term gestation and spontaneous onset of labor (Group 1, OR 2.91, 95% CI 2.87 to 2.96, p < 0.001; S2 Table).

Table 2

The Robson Classification Report Table [9], Sweden and BC, Canada, 2004–2016.

	CDs No.		All deliveries No.		Relative size (%)		CD rate (%)		Absolute contribution to overall CD rate (%)		Relative contribution to overall CD rate (%)
Robson group	Sweden	BC	Sweden	BC	Sweden	BC	Sweden	BC	Sweden	BC	Sweden	BC
1. Nulliparous, singleton, cephalic, ≥37 weeks, spontaneous labor	34,884	30,848	431,199	151,106	31.0	27.0	8.1	20.4	2.5	5.5	14.5	17.7
2. Nulliparous, singleton, cephalic, ≥37 weeks, induced or CD before labor	42,474	31,255	113,927	68,084	8.2	12.2	37.3	45.9	3.0	5.6	17.6	17.9
2a. Induced	26,623	24,491	98,076	61,320	7.0	11.0	27.2	39.9	1.9	4.4	11.0	14.0
2b. CD before labor	15,851	6,764	15,851	6,764	1.1	1.2	100.0	100.0	1.1	1.2	6.6	3.9
3. Parous, singleton, cephalic, ≥37 weeks, no previous CD, spontaneous labor	8,114	3,910	500,236	151,071	35.9	27.0	1.6	2.6	0.6	0.7	3.4	2.2
4. Parous, singleton, cephalic, ≥37 weeks, no previous CD, induced or CD before labor	21,296	5,328	98,901	40,740	7.1	7.3	21.5	13.1	1.5	1.0	8.8	3.1
4a. Induced	4,461	2,656	82,066	38,068	5.9	6.8	5.4	7.0	0.3	0.5	1.8	1.5
4b. CD before labor	16,835	2,672	16,835	2,672	1.2	0.5	100.0	100.0	1.2	0.5	7.0	1.5
5. Parous, singleton, cephalic, ≥37 weeks, with a previous CD	61,923	58,277	120,104	71,665	8.6	12.8	51.6	81.3	4.5	10.4	25.7	33.4
6. Nulliparous, singleton, breech	25,804	12,351	27,503	12,932	2.0	2.3	93.8	95.5	1.9	2.2	10.7	7.1
7. Parous, singleton, breech	14,661	7,908	16,572	8,771	1.2	1.6	88.5	90.2	1.1	1.4	6.1	4.5
8. Multiple pregnancy (twins or higher-order multiples)	10,780	6,244	19,701	8,972	1.4	1.6	54.7	69.6	0.8	1.1	4.5	3.6
9. Singleton, transverse or oblique lie	1,892	1,731	1,905	1,828	0.1	0.3	99.3	94.7	0.1	0.3	0.8	1.0
10. Singleton, cephalic, <37 weeks	17,178	11,699	58,500	37,968	4.2	6.8	29.4	30.8	1.2	2.1	7.1	6.7
Unknown*	2,130	4,904	4,231	6,068	0.3	1.0	50.3	80.8	0.2	0.9	0.9	2.8
Total obstetric population	241,136	174,455	1,392,779	559,205	100.0	100.0	17.3	31.2	17.3	31.2	100	100

*All remaining records that could not be classified due to missing information on 1 or more of the following variables: fetal presentation, parity, gestational age, type of labor, or previous CD.

BC, British Columbia; CD, cesarean delivery; No., number.

Temporal trends in CD rates by Robson group

In Sweden, the overall rate of CD remained stable from 16.8% in 2004 to 17.6% in 2016 (P for trend = 0.1; S3 Table). Notable changes by Robson group included a decline in CDs among nulliparous women with a term, singleton fetus in cephalic presentation with induced labor (Group 2a; 28.6% to 24.3%), and an increase in CD from 49.4% to 52.8% among women with a previous CD (Group 5). In contrast, CD rates increased substantially during this period in BC. Overall, the rate of CD increased from 29.4% to 34.0% (P for trend < 0.0001; S4 Table). The CD rate in BC increased from 37.5% to 45.4% in Robson Group 2a and from 26.7% to 37.6% in women who delivered preterm (Group 10). Temporal trends in the rate of CD, the relative size, and the relative contribution to the overall CD rate among select Robson groups of interest are compared in Fig 2 in Sweden and BC.

Fig 2

Rate of CD (panel A), relative size (panel B), and contribution to the overall CD rate (panel C) among selected Robson groups, Sweden, and BC, Canada, 2004–2016.

BC, British Columbia; CD, cesarean delivery.

Contribution of each Robson group to the overall rate of CD

In both Swedish and BC cohorts, the same 3 Robson groups showed the largest contribution to the overall rate of CD (Table 2). The largest contributing group was Robson Group 5, women with at least 1 previous CD and a term, singleton, cephalic-presenting fetus. Although this group accounted for only 8.6% and 12.8% of the total obstetric population in Sweden and BC, respectively, the high rate of CD in these women (51.6% in Sweden; 81.3% in BC) made this group responsible for approximately 1 in 4 CDs in Sweden and 1 in 3 CDs in BC. Robson Group 1 (nulliparous women with a single, cephalic-presenting fetus at term gestation, and spontaneous labor) made the second largest absolute contribution to the overall rate of CD (2.5% in Sweden and 5.5% in BC). Group 2a (nulliparous women with a single, cephalic-presenting fetus at term gestation, and induced labor) made the third largest absolute contribution to the overall CD rate (1.9% in Sweden and 4.4% in BC). CDs in the abovementioned Robson groups (5, 1, and 2a) were responsible for 82.1% (11.5% of 14.0%) of the excess CDs in BC compared with Sweden (Fig 3).

Fig 3

Cumulative difference in rate of CD (per 100 deliveries) in BC, Canada vs. Sweden, by Robson group, 2004–2016.

BC, British Columbia; CD, cesarean delivery.

Contribution of maternal, fetal, and obstetric practice factors to spatial differences in CD rates

The distribution of risk factors for CD in Sweden and BC by Robson group are included in S5–S16 Tables. In general, deliveries to women with advanced maternal age, labor induction, and CD before labor were more common in BC, while spontaneous onset of labor and macrosomic infants were more common in Sweden (Table 1). The rate of preeclampsia was higher in Sweden compared with BC in all Robson groups (S1 Fig), and the proportion of women delivering at 42 weeks’ gestation or beyond was higher among Swedish women, particularly in Group 2a (31.4% in Sweden versus 3.0% in BC). However, most women who delivered at postterm gestation in Sweden delivered by 42 + 2 (50%) and 42 + 3 (75%) weeks’ gestation.

After adjustment for maternal, fetal, and obstetric practice factors not represented in the Robson scheme, the rate of CD in BC compared with Sweden was significantly attenuated in Groups 1, 2, 4, 5, and 8 and significantly increased in Group 10 (Table 3). Adjustment explained 14% of the increase in CD in BC versus Sweden in Robson Group 2 (RR = 1.23, 95% CI 1.22 to 1.25, p < 0.001; ARR = 1.09, 95% CI 1.08 to 1.12, p < 0.001) and 10.0% of the increase in CD in BC versus Sweden in Robson Group 5 (RR = 1.58, 95% CI 1.57 to 1.59, p < 0.001; ARR = 1.48, 95% CI 1.47 to 1.50, p < 0.001), which corresponded to a 5% absolute reduction in the adjusted CD rate in BC in both groups. Notably, the difference in the rate of CD between BC and Sweden widened by 30.8% after adjustment in Group 4, the only group in which the crude rate of CD was substantially lower in BC compared with Sweden (Table 3).

Table 3

Crude and adjusted rates and rate ratios for CD in BC vs. Sweden by Robson group, 2004–2016.

Robson group	Crude CD rate (%)		CD BC vs. Sweden				Adjusted CD rate in BC (%)‡
	Sweden	BC	RR (95% CI)	P-value*	ARR† (95% CI)	P-value*
1	8.1	20.4	2.52 (2.49–2.56)	<0.001	2.32 (2.29–2.36)a,b	<0.001	18.8
2	37.3	45.9	1.23 (1.22–1.25)	<0.001	1.09 (1.08–1.12)b	<0.001	40.7
3	1.6	2.6	1.60 (1.54–1.66)	<0.001	1.54 (1.48–1.60)a	<0.001	2.5
4	21.5	13.1	0.61 (0.59–0.62)	<0.001	0.49 (0.48–0.51)	<0.001	10.5
5	51.6	81.3	1.58 (1.57–1.59)	<0.001	1.48 (1.47–1.50)	<0.001	76.4
6	93.8	95.5	1.02 (1.01–1.02)	<0.001	1.02 (1.01–1.02)b,c	0.02	95.5
7	88.5	90.2	1.02 (1.01–1.03)	<0.001	1.02 (1.01–1.03)c	0.03	90.2
8	54.7	69.6	1.27 (1.25–1.30)	<0.001	1.19 (1.17–1.22)c	<0.001	65.1
9	99.3	94.7	0.95 (0.94–0.96)	<0.001	0.95 (0.94–0.97)c	0.03	94.7
10	29.4	30.8	1.04 (1.03–1.07)	<0.001	1.11 (1.09–1.14)d	<0.001	32.6
All groups	17.3	31.2	1.80 (1.79–1.81)	<0.001

*P-values represent significance of Wald chi-square test; the a priori level of statistical significance was set at a 2-sided p-value < 0.05.

†Adjusted models included maternal age, parity, early-pregnancy BMI, smoking during pregnancy, chronic hypertension, preexisting diabetes, in vitro fertilization, preeclampsia/eclampsia, postterm delivery, position of the fetal head at delivery, infant birth weight, infant head circumference, and congenital anomaly.

‡Adjusted CD rate in BC = crude CD rate in Sweden*ARR.

aAdjusted model also included epidural anesthesia.

bAdjusted model excluded parity due to group restriction to nulliparas.

cAdjusted model excluded position of the fetal head at delivery due to group restriction to non-cephalic fetal presentation or multiple gestation.

dAdjusted model excluded postterm delivery due to group restriction to preterm deliveries.

ARR, adjusted rate ratio; BC, British Columbia; BMI, body mass index; CD, cesarean delivery; CI, confidence interval; RR, rate ratio.

Contribution of maternal, fetal, and obstetric practice factors to temporal changes in CD rates

Changes in maternal, fetal, and obstetric practice factors over the study period are presented for Robson Groups 1, 2a, and 5, as these groups contributed most to the overall CD rates in BC and Sweden (S17–S19 Tables and S2–S4 Figs). Early-pregnancy BMI increased and smoking rates declined across all groups of women in both countries. Advanced maternal age (35+ years) increased, particularly in BC (e.g., from 19.4% in 2004 to 25.8% in 2016 in Robson Group 2a; S18 Table), while the rate of preeclampsia in Sweden decreased significantly in all 3 groups over the study period.

The crude CD rate among women in Robson Group 1 declined in Sweden from 8.1% in 2004 to 7.7% in 2016 (RR 2014 to 2016 versus 2004 to 2007 = 0.93, 95% CI 0.91 to 0.96, p < 0.001), and this rate was further attenuated after adjustment for maternal, infant, and obstetric practice factors (Table 4). In BC, the crude rate of CD in Robson Group 1 increased from 19.7% in 2004 to 22.6% in 2016 (RR 2014 to 2016 versus 2004 to 2007 = 1.12, 95% CI 1.09 to 1.16, p < 0.001). The observed increase in CD was entirely explained by adjustment for changes in maternal characteristics and obstetric practice factors (ARR 2014 to 2016 versus 2004 to 2007 = 1.01, 95% CI 0.99 to 1.04, p = 0.80; Table 4 and Fig 4).

Table 4

Unadjusted and adjusted rate ratios for CD in 2014–2016 vs. 2004–2007 among women in Robson Groups 1, 2a, and 5 after sequential adjustment* for maternal characteristics, obstetric practice factors, and fetal/infant characteristics, Sweden and BC, Canada.

Determinants adjusted for	Robson Group 1				Robson Group 2a				Robson Group 5
	Sweden		BC		Sweden		BC		Sweden		BC
	RR(95% CI)	P-value†	RR(95% CI)	P-value†	RR(95% CI)	P-value†	RR(95% CI)	P-value†	RR(95% CI)	P-value†	RR(95% CI)	P-value†
Unadjusted	0.93(0.91–0.96)	<0.001	1.12(1.09–1.16)	<0.001	0.84(0.82–0.87)	<0.001	1.18(1.15–1.21)	<0.001	1.05(1.03–1.06)	<0.001	0.98(0.97–0.99)	0.01
Adjusted for maternal characteristicsa	0.92(0.90–0.95)	<0.001	1.06(1.03–1.09)	<0.001	0.84(0.82–0.87)	<0.001	1.11(1.08–1.14)	<0.001	1.05(1.03–1.06)	<0.001	0.98(0.97–0.99)	0.03
Also adjusted for maternal conditionsb	0.93(0.90–0.95)	<0.001	1.06(1.03–1.09)	<0.001	0.85(0.82–0.87)	<0.001	1.11(1.08–1.14)	<0.001	1.04(1.03–1.06)	<0.001	0.98(0.97–0.99)	0.04
Also adjusted for obstetric practice factorsc	0.89(0.86–0.91)	<0.001	1.01(0.98–1.03)	0.74	0.85(0.82–0.87)	<0.001	1.09(1.06–1.12)	<0.001	1.04(1.03–1.06)	<0.001	0.98(0.97–0.99)	0.04
Also adjusted for fetal/infant characteristicsd	0.91(0.88–0.93)	<0.001	1.01(0.99–1.04)	0.80	0.86(0.84–0.88)	<0.001	1.09(1.07–1.13)	<0.001	1.04(1.03–1.06)	<0.001	0.97(0.96–0.98)	0.03

*Sequential adjustment was carried out by fitting a series of models with additional groups of factors added to each model in the sequence outlined above to quantify the contribution of each group of factors to CD trends over time.

†P-values represent significance of Wald chi-square test; the a priori level of statistical significance was set at a 2-sided p-value < 0.05.

aMaternal characteristics included maternal age, early-pregnancy BMI, smoking during pregnancy, and parity (only for Group 5 since Group 1 and 2a are restricted to nulliparous women).

bMaternal conditions included preeclampsia/eclampsia, preexisting diabetes, in vitro fertilization, and chronic hypertension.

cObstetric practice factors included postterm delivery and epidural anesthesia (in Groups 1 and 2a only).

dFetal/infant characteristics included position of the fetal head at delivery, infant birth weight, infant head circumference, and congenital anomaly.

BC, British Columbia; CD, cesarean delivery; CI, confidence interval; RR, rate ratio.

Fig 4

Observed rates of CD and rates adjusted sequentially for changes in maternal characteristics, maternal conditions, obstetric practice, and fetal/infant characteristics among women in Robson Group 1 (panel A), Robson Group 2a (panel B), and Robson Group 5 (panel C) in Sweden and BC, Canada, 2004–2016.

Note that the y axis scales differ for the presentations of Robson Groups 1, 2a, and 5. BC, British Columbia; CD, cesarean delivery.

Among women in Robson Group 2a, the crude rate of CD decreased from 28.6% in 2004 to 24.3% in 2016 in Sweden and increased from 37.5% to 45.4% in BC over the same period. Adjustment did not affect the trend in the rate of CD in this group in Sweden (Fig 4). In BC, adjustment for changes in maternal characteristics (mainly age) explained 7% of the 18% relative increase in CD over this period (Table 4). Nevertheless, the upward trend in CD by period remained significant after sequential adjustment for all determinants (ARR 2014 to 2016 versus 2004 to 2007 = 1.09, 95% CI 1.07 to 1.13, p < 0.001; Table 4).

In Sweden, the crude rate of CD in Robson Group 5 increased from 49.4% in 2004 to 52.8% in 2016 (RR 2014 to 2016 versus 2004 to 2007 = 1.05, 95% CI 1.03 to 1.06, p < 0.001) and the increase was unaffected by adjustment (Table 4 and Fig 4). The effect of adjustment for each group of risk factors by period in Groups 1, 2a, and 5 is tabulated for both cohorts in S20 Table.

After restricting deliveries in BC to those with midwifery care, the overall rate of CD in BC was 19.9%. The main differences in the rates of CD in this restricted group compared with all deliveries in BC was a lower rate of CD in women with a previous CD (Group 5), women with multiple pregnancies (Group 8), and those who delivered preterm (Group 10; S21 Table). Among these women as well, adjustment for maternal, fetal, and obstetric practice factors attenuated the difference in CD rates in BC versus Sweden in Groups 1, 2, 4, and 10 but not in Groups 5 and 8 (S22 Table).

Analyses restricted to individuals with no missing information on BMI and models with missing BMI values imputed yielded almost identical results for comparisons of CD rates between countries and within countries over time (S23 and S24 Tables).

Discussion

We applied the Robson classification to 1,951,984 deliveries between 2004 and 2016 in Sweden and BC, Canada. The effect of controlling for maternal, fetal, and obstetric factors on the spatial comparisons differed by Robson group and ranged from no effect (e.g., among deliveries with breech presentation; Groups 6 and 7) to explaining up to 5.2% of the absolute 8.6% difference in CD rates in BC versus Sweden in Group 2 (nulliparous women with a term, cephalic fetus with induced labor, or a CD without labor). Nonetheless, wide spatial differences in CD rates persisted even after adjustment, especially in nulliparous women with a term, cephalic fetus with spontaneous labor, and women with a previous CD (Robson Groups 1 and 5, respectively). Adjustment for maternal, fetal, and obstetric practice factors explained a substantial fraction of the temporal change in CD rates in BC in Robson Groups 1 and 2a but had little impact on the temporal changes in CD rates observed among specific Robson groups in Sweden (such as Groups 2a and 5).

Several studies of obstetric populations in high-income settings have found that Robson Groups 1, 2a, and 5 contribute the most to overall CD rates [3-6], and the distribution of deliveries across Robson groups in both Sweden and BC is consistent with expected relative group sizes [33]. To our knowledge, there are no studies that have quantified international variation in CD rates using the Robson classification with and without adjustment for differences in maternal, fetal, and obstetric practice factors. One analysis that compared CD rates across 4 Nordic countries using the Robson classification adjusted solely for maternal age and did not report the variation in crude and adjusted estimates of CD [5]. The few studies that have combined Robson stratification and regression to estimate temporal variation in CD rates have reported mixed results. Similar to our findings, attenuation in CD rates over time after case-mix adjustment was observed among women in Robson Groups 1 and 3 in analyses from northern Italy [17,18]. On the other hand, a population-based study in France found that an increase in CD in Robson Group 1 from 2003 to 2010 did not change after adjustment for risk factors for CD, although this may be explained by limitations in available information as the models did not include BMI, maternal comorbidity, or fetal characteristics [19].

The strengths of our study include the ability to examine a large number of deliveries owing to the use of 13 years of data from population-based birth registers in 2 countries. In addition, we were able to include important determinants of CD in our analyses, such as maternal comorbidity, maternal BMI, fetal macrosomia, and position of the fetal head at delivery that have been excluded in previous analyses. The data sources used have been shown to be accurate in validation studies [24,25], and the proportion of women left uncategorized by Robson classification due to missing data was low (<1.0%).

The limitations of our study include potential data transcription errors and omissions in coding, which are inevitable in large health databases. Further, 13.8% of women (7.9% in Sweden and 28.3% in BC) in our study had missing values for early-pregnancy BMI and we included these women in our main models using a missing indicator approach. However, complete-case analyses (excluding cases with missing BMI values) and analyses including imputed values for missing BMI yielded similar results. Since we do not know if the BMI distribution was different among those with missing versus available BMI information, it is challenging to speculate on the direction and magnitude of bias introduced by missing data for BMI [34]. Confirmation of our findings in a cohort with complete BMI data would be valuable. The relatively large size of Group 4b (multiparous women with prelabor CD) in Sweden may indicate misclassification of women with a previous CD (i.e., they should have been assigned to Group 5); alternatively, this may indicate higher rates of CD on maternal request due to a variety of reasons, such as previously traumatic or prolonged labor. Finally, we were not able to account for important intrapartum details such as indication for CD, duration of labor, details regarding oxytocin augmentation, and fetal surveillance because our data sources lacked such information.

We quantified the crude and adjusted temporal trend in CD in a commonly used subdivision of Robson Group 2 restricted to women who had an induction (Group 2a) to increase the homogeneity of this group before assessing the contribution of maternal, fetal, and obstetric practice characteristics on temporal trends in CD. However, it is important to note that the analysis of any subdivision alone may be misleading without a consideration of the other subdivisions that compose the complete group (Group 2b in this case). In our analyses, the exclusion of Group 2b is unlikely to have had a substantial effect on temporal changes in the CD rate in Group 2 since the relative size of Group 2b was small (1.1% and 1.2% in Sweden and BC, respectively) and remained small throughout the study period. The relative size and high rate of CD in Group 2a are the reasons for this group’s position as one of the 3 largest contributors to the overall CD rate in both Sweden and BC.

While our study explained spatiotemporal variations in CD rates, it did not address the perinatal and maternal morbidity and mortality that was caused or prevented by the observed differences in CD rates. Future studies are required to evaluate the differences in CD rates in each Robson group within the context of maternal, fetal, and infant morbidity and mortality. Our findings reinforce that, although often misinterpreted, the Robson classification was not meant to be an endpoint of CD comparison, but rather a starting point or initial framework within which determinants and outcomes of CD can be analyzed [10]. These results also highlight the importance of comprehensive and accurate perinatal data collection to ensure public health initiatives regarding CD rates are suitably evidence informed and prioritized. The maternal, obstetric practice, and fetal/infant characteristics included in our analyses were not exhaustive, and it is possible that the inclusion of other factors not represented in the Robson classification scheme (e.g., duration of labor, indication for CD) could have even larger impacts on CD rates.

The simplicity and efficiency of the Robson classification system has resulted in its endorsement by international health organizations and expansive uptake worldwide. However, our analyses show that maternal, fetal, and obstetric practice factors not included in the Robson classification explain a significant proportion of the spatiotemporal difference in CD rates in some Robson groups and should be incorporated into explanatory models evaluating CD rates in populations. Public health initiatives based on Robson-classified CD rates may lead to erroneous attributions of variation in CD rates to differences in obstetric practice without a comprehensive consideration of relevant maternal, obstetric, and fetal factors.

Study protocol.

Protocol for “Epidemiologic evaluation of cesarean delivery trends in Canada and Sweden” study.

(DOCX)

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Checklist of items that should be included in reports of observational studies.

(DOCX)

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Assessment of quality of data, type of population, and cesarean delivery rates.

Steps defined by the World Health Organization to assess quality of data, type of population, and cesarean delivery rates using the Robson classification.

(DOCX)

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Crude and adjusted rates and odds ratios for cesarean delivery in British Columbia vs. Sweden by Robson group, 2004–2016.

To quantify comparisons of cesarean delivery rates in British Columbia vs. Sweden between Robson groups, we calculated odds ratios and 95% confidence intervals for Robson group-specific CD rates since the odds ratio (unlike the rate ratio) is not susceptible to artefactual constraints when the baseline CD rate is high.

(DOCX)

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Cesarean delivery rate by year of delivery and Robson group, Sweden, 2004–2016.

Robson group-specific cesarean delivery rates by year, percent change in cesarean delivery rates, and p-value for linear trend over the study period in Sweden.

(DOCX)

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Cesarean delivery rate by year of delivery and Robson group, British Columbia, Canada, 2004–2016.

Robson group-specific cesarean delivery rates by year, percent change in cesarean delivery rates, and p-value for linear trend over the study period in British Columbia.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 1, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 1.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 2a, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 2a.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 2b, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 2b.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 3, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 3.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 4a, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 4a.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 4b, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 4b.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 5, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 5.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 6, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 6.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 7, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 7.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 8, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 8.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 9, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 9.

(DOCX)

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Maternal, obstetric practice, and fetal/infant characteristics in deliveries among women in Robson Group 10, Sweden and British Columbia, Canada, 2004–2016.

Distribution of determinants of cesarean delivery in Robson Group 10.

(DOCX)

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Frequency, proportion and rate ratio of maternal, obstetric practice, and fetal/infant characteristics among deliveries to women in Robson Group 1 in 2014–2016 vs. 2004–2007, Sweden and British Columbia.

Estimates of temporal trends in determinants of cesarean delivery in Robson Group 1.

(DOCX)

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Frequency, proportion and rate ratio of maternal, obstetric practice, and fetal/infant characteristics among deliveries to women in Robson Group 2a in 2014–2016 vs. 2004–2007, Sweden and British Columbia.

Estimates of temporal trends in determinants of cesarean delivery in Robson Group 2a.

(DOCX)

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Frequency, proportion and rate ratio of maternal, obstetric practice, and fetal/infant characteristics among deliveries to women in Robson Group 5 in 2014–2016 vs. 2004–2007, Sweden and British Columbia.

Estimates of temporal trends in determinants of cesarean delivery in Robson Group 5.

(DOCX)

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Crude and adjusted rate ratios for cesarean delivery in 2014–2016 vs. 2004–2007 among women in Robson Groups 1, 2a, and 5 after sequential adjustment for maternal characteristics, obstetric practice factors, and fetal/infant characteristics, Sweden and British Columbia, Canada.

Comparing the crude and adjusted differences in cesarean delivery rates by country and study period in Robson Groups 1, 2a, and 5.

(DOCX)

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Rate of cesarean delivery by Robson classification groups among women with midwifery care, British Columbia, Canada, 2004–2016.

Distribution of deliveries and cesarean deliveries restricted to women in British Columbia with midwifery-led maternity care.

(DOCX)

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Crude and adjusted rates and rate ratios for cesarean delivery among women with midwifery care, British Columbia, Canada, vs. Sweden, by Robson group, 2004–2016.

Comparing Robson group-specific cesarean delivery rates by country restricted to women in British Columbia with midwifery-led maternity care.

(DOCX)

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Crude and adjusted rates and rate ratios for cesarean delivery, British Columbia, Canada vs. Sweden, by Robson group, 2004–2016.

Comparing cesarean delivery rates in British Columbia vs. Sweden. (A) Excluding deliveries with missing values for body mass index and (B) using multiple imputation for missing body mass index values.

(DOCX)

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Crude and sequentially adjusted rate ratios of cesarean delivery rates in 2014–16 vs. 2004–2007 in Robson Groups 1, 2a, and 5, Sweden and British Columbia, Canada.

Comparing crude and adjusted temporal changes in the cesarean delivery rate by country. (A) Excluding deliveries with missing values for body mass index, and (B) using multiple imputation for missing body mass index values.

(DOCX)

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Rate of preeclampsia/eclampsia by Robson group, Sweden and British Columbia, 2004–2016.

Proportion of women diagnosed with preeclampsia/eclampsia stratified by country and Robson group. Sweden is represented in the blue bars and BC is represented in the orange bars. The error bars indicate the 95% confidence interval. “RG” denotes Robson group, “Unk” denotes unknown.

(PDF)

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Temporal trends in maternal characteristics, obstetric practice factors, and fetal/infant characteristics among women in Robson Group 1, Sweden and British Columbia, 2004–2016.

Changes in the frequency of determinants of cesarean delivery over the study period in Robson Group 1.

(PDF)

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Temporal trends in maternal characteristics, obstetric practice factors, and fetal/infant characteristics among women in Robson Group 2a, Sweden and British Columbia, 2004–2016.

Changes in the frequency of determinants of cesarean delivery over the study period in Robson Group 2a.

(PDF)

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Temporal trends in maternal characteristics, obstetric practice factors, and fetal/infant characteristics among women in Robson Group 5, Sweden and British Columbia, 2004–2016.

Changes in the frequency of determinants of cesarean delivery over the study period in Robson Group 5.

(PDF)

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24 Jan 2022

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PLOS Medicine

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Requests from the editors:

General comments:

Throughout the paper, please adapt reference call-outs to the following style: "... gestational age (<37 vs ≥37 weeks) [8,9]." (noting the absence of spaces within the square brackets).

Data availability:

The Data Availability Statement (DAS) requires revision. If the data are owned by a third party but not freely available upon request, please state the owner of the data set and contact information for data requests (web or email address). Note that a study author cannot be the contact person for the data.

Title: Please revise your title according to PLOS Medicine's style. We suggest “Adjusted comparisons of cesarean delivery rates between populations using the Robson classification: A population-based cohort study in Canada and Sweden, 2004-2016” or similar

Abstract:

Please structure your abstract using the PLOS Medicine headings (Background, Methods and Findings, Conclusions).

Abstract Background: The final sentence should clearly state the study question.

Line 36 - ‘cesarean delivery’ is duplicated

Abstract Methods and Findings:

Please provide brief demographic details of the study population (e.g. fetal sex, maternal age, ethnicity, etc). Please also specify that data used in the analysis are from January 1, 2004 and December 31, 2016

Please define CD and BC at first use

Please define Group 2

Please include the most important variables that are adjusted for in the analyses.

Please include the actual amounts and/or absolute risk(s) of relevant outcomes, not just RRs.

In the last sentence of the Abstract Methods and Findings section, please describe 2-3 of the main limitations of the study's methodology.

Abstract Conclusions:

Please begin your Abstract Conclusions with "In this study, we observed ..." or similar, to summarize the main findings from your study, without overstating your conclusions. Please emphasize what is new and address the implications of your study, being careful to avoid assertions of primacy.

Author Summary:

At this stage, we ask that you include a short, non-technical Author Summary of your research to make findings accessible to a wide audience that includes both scientists and non-scientists. The Author Summary should immediately follow the Abstract in your revised manuscript. This text is subject to editorial change and should be distinct from the scientific abstract. Please see our author guidelines for more information: https://journals.plos.org/plosmedicine/s/revising-your-manuscript#loc-author-summary

In the final bullet point of ‘What Do These Findings Mean?’, please describe the main limitations of the study in non-technical language.

Methods:

Did your study have a prospective protocol or analysis plan? Please state this (either way) early in the Methods section. If a prospective analysis plan (from your funding proposal, IRB or other ethics committee submission, study protocol, or other planning document written before analyzing the data) was used in designing the study, please include the relevant prospectively written document with your revised manuscript as a Supporting Information file to be published alongside your study, and cite it in the Methods section. A legend for this file should be included at the end of your manuscript. If no such document exists, please make sure that the Methods section transparently describes when analyses were planned, and if/when reported analyses differed from those that were planned. Changes in the analysis-- including those made in response to peer review comments-- should be identified as such in the Methods section of the paper, with rationale. If a reported analysis was performed based on an interesting but unanticipated pattern in the data, please be clear that the analysis was data-driven.

Thank you for providing a STROBE checklist as Supporting Information. Please add the following statement, or similar, to the Methods: "This study is reported as per the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline (S1 Checklist)." The STROBE guideline can be found here: http://www.equator-network.org/reporting-guidelines/strobe/ When completing the checklist, please use section and paragraph numbers, rather than page numbers which will likely no longer correspond to the appropriate sections after copy-editing.

Line 308 - Please replace "subject" with participant, patient, individual, or person.

Results:

Please incorporate table S4 into the main paper as Table 1, to show the baseline characteristics of the study population.

Line 258 - Please revise use of ‘significant’ unless statistical analyses are provided to substantiate statistical significance

Discussion:

Please rename your ‘Comment’ section to ‘Discussion’, and remove all subheadings e.g. ‘Principal findings’ and ‘Conclusion’

Please present and organize the Discussion as follows: a short, clear summary of the article's findings; what the study adds to existing research and where and why the results may differ from previous research; strengths and limitations of the study; implications and next steps for research, clinical practice, and/or public policy; one-paragraph conclusion.

Tables:

Please define all abbreviations used in the table legend of each table.

References:

Please ensure that journal name abbreviations match those found in the National Center for Biotechnology Information (NCBI) databases (http://www.ncbi.nlm.nih.gov/nlmcatalog/journals), and are appropriately formatted and capitalised. Please also name 6 authors prior to ‘et al’.

Where website addresses are cited, please specify the date of access.

Comments from the reviewers:

Reviewer #1: Thanks for the opportunity to review your manuscript. My role is as a statistical reviewer so my comments focus on the study design, data, and analysis presented in the manuscript. I have put general comments first, and followed these with queries relevant to specific section of the manuscript (with a line reference).

This study uses data pooled from national data from Sweden and province level data from BC in Canada (from the birth registration systems). This manuscript evaluates if the standard for classification of deliveries used for comparing caesarean delivery rates can be improved by considering other maternal/fetal/health service characteristics that could explain variation in CDR. The study is large (~2 milliion deliveries) and aside from a few variables the data appears fairly complete. There are some queries below but the manuscript is written clearly and addresses an interesting question.

My background is not in working in perinatal epidemiology, and I was curious about how much of the overall rates in CD appear to come from a few specific Robson groups. Does the higher rates reflect a genuine healthcare need for these mothers, or is it from failure of the healthcare services to avoid an unnecessary CD? (this is question much more motivated by my own interest rather than a need to change the manuscript)

One thing I think needs some rewording is the description of shift in RR after including additional covariates in the models. Typically ICC (and changes in) would be used a metric of variation and shift in unexplained variation, but to my knowledge this cannot be done with the modified Poisson regression models used. I think the approach of examining how the estimated RRs shift with additional adjustment is reasonable - but I think would be better simply described as absolute shift in estimated rate ratio to the specific Robson group/countries.

L55. I am not familiar with the 'sequential' terminology - is this an extension of the Generalised Linear Model?

L140. Is this extension of the Robson classification routinely used?

L155. Are these comorbidity categories examples of the ones used, or the complete set?

L166. I have used GEE procedures with the modified Poisson regression to get estimates of rate ratios/relative risk (i.e. the robust standard error), in these cases each row/observation of data was independent so the independent working structure matched the data. If there are multiple deliveries per mother in the data, is an independent working structure realistic, i.e. compared to an exchangeable/CS structure?

L171. This does assume that there is no correlation of missing BMI status with other covariates. This is assessed later on (seems to be fine), I would describe this approach as 'missing as indicator' in the methods. I would also specifically say you completed a complete-case analysis with respect to the other variables. I wasn't clear how many from each country were excluded on this basis - it would be helpful to have this detail in a flow diagram describing how many deliveries were initially included and then what remained after any exclusions for missing data. Was there variation in rates of missingness (in BMI and the other variables) over time?

L208. How did you decide on the selected number of imputations?

L286. Just checking the estimate and 95% CI here: 1.01 (0.99-1.04). This looks a bit odd - maybe just from rounding?

Appendix - Table S23. Why did some of the adjusted models exclude particular variables? Because of small cell sizes?

Reviewer #2: Thanks for opportunity to review this paper.

* This is a novel and well-conducted analysis of retrospective birth data in 2 countries. The analysis addresses a well-known thorny issue in Robson-based analyses of CS usage. The methods are well-described.

* Did the authors explore the possibility of differences in the validity/reliability of the 2 datasets, particularly for Robson variables, which might have contributed to inter-country differences?

* Line 158 - The consideration of 2 obstetric practice-related factors (Epidural anesthesia and post-term delivery) seemed somewhat brief to me and a potential major limitation. A number of practice or care-setting-related variables have been shown to be associated with CS rates, including but not limited to level of care (secondary vs tertiary), private vs public, antenatal care provision/provider, presence/absence of labour companion, continuity of care provider, VBAC policies, mandatory second opinion and others. I note that presence/absence of midwife-led care is dealt with through sensitivity analysis. I appreciate several of these are probably not be available in the individual level data, but it is necessary for the authors to explain the rationale for selection of only these 2 variables, and why others were not included or considered.

* 203 - The issue of BMI driving CS rates is one of significant debate. Some clarification on the timing of early pregnancy BMI measurement would be useful. Also, 28% seems quite high for a missing rate and I did wonder if these are missing at random or not, and whether multiple imputation is appropriate in this context.

Reviewer #3: This paper reports the findings of an innovative approach to contribute to the understanding of the CD epidemiology that is traditionally done using the well recognized and globally validated Robson classification. The authors add maternal, fetal and obstetrical practice factors to the Robson classifications in Sweden and BC.

The paper is well written, well established methodology taken into account models of obstetrical care, and reports on a huge sample size over a 12 years period of time, analyzing population based data sets. The findings are important, significant and show the weight of mainly maternal factors adding value to the Robson classification for clinical practice.

One question however remains: the constant lower and stable CD rates in Sweden as compared to the higher and increasing rates in BC can not be fully explained by differences in maternal or obstet practice factors. Did the authors have the opportunity to compare the CD rates in BC between midwifery and ob/gyn led models? or financial/ cost factors in the health care models of both sites?

Reviewer #4: This is a very interesting paper of almost 2 million deliveries between 2004 and 2016 in Sweden and BC Canada. It significantly adds to the literature. As the authors discuss, there has been a significant amount of work published on the Robson Ten Group Classification system, but the effect of controlling for maternal, obstetric and fetal factors the way the authors do in this work is novel.

I have one comment in relation to lines 218 - 220

"The rate of CD was higher in BC compared with Sweden in all Robson Groups except in Group 4 (parous women with a single, cephalic fetus at term gestation with an induction or CD prior to

labour) and among deliveries with a transverse/oblique-presenting fetus (Group 9)"

Group 9 is often used as a quality control mechanism for the data and the CS rate in this group should be 100%. Therefore, it may be reflective of the quality of the data collection rather than an actual difference itself.

Notwithstanding this minor point, I believe that this work is very worthy of publication and gives a very clear insight into the significant spatio-temporal differences in certain Robson groups.

Reviewer #5: Thank you for the opportunity to review your paper and i appreciate all the hard work that has gone into it.

I would like to make the following points.

The TGCS has never been intended to be anything more than a starting point and a structure within which all other epidemiological variables, differences in practice and events and outcomes are analysed. I am not sure that the readers of the paper will understand that when you discuss it in the paper.

I would always start by using the standard TGCS table first. There are a number of reasons why this is strongly advised

I get the impression from reading it that you have used data that is available rather than comment on the unavailability/quality of important information that should be there and if not that is the surely the big problem? So the TGCS structure is accurately collected (less then 1% of women could not be classified) but no real information on perinatal (in particular neonatal) outcomes or labour and delivery practice. Even BMI was not available in 28 % of women in BC.

There was no information on oxytocin rates, indications for CS or indications for pre-labour CS

I feel that there is a simple message from your TGCS data and that is that there are significant differences in CS rates between Sweden and BC. There are also more conclusions/hypotheses that you could have made from the raw TGCS table but you did not and i would be happy to help with that. With a good classification system there are only 3 reasons why there is a difference in either the sizes of the groups or the incidence of events within the groups. They are data quality, significant epidemiological variables and lastly differences in practice. By far the most common reason in perinatal data is data quality (including definitions)

While i absolutely agree with you that that the TGCS is only a structure and to make further conclusions in validating the difference between CS rates you need more information I am not sure with the limited data that you have presented that we are any further forward.

The real point that your paper shows is how poor we all are in collecting routine perinatal data either because of a lack of resources or/and discipline together with a complete lack and agreement of further classifications. For example indications for CSs and inductions. Also your comment 369-371 supports this referring to lack of information on oxytocin, length of labour and fetal surveillance.

As far as the temporal and spatial differences in CS rates it is clear in the standard description of TGCS methodology that before considering the differences in CS rates you must examine any changes in the sizes of the groups first over the same period of time. You made no comment on that and even changed the design of the TGCS table by placing the column indicating the sizes of the group after the column showing the CS rates in each of the groups. This is confusing to readers who use the TGCS when each new paper invents their own way of designing the TGCS table

These comments are meant to be constructive and i think you should rethink what the main conclusion of what all your hard work has really shown especially the deficiencies. I would be happy to help in any way i can

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[LINK]

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Submitted filename: Known knowns, unknown unknowns a.pdf

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Submitted filename: renamed_e5be2.pdf

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23 May 2022

Submitted filename: Response to Review 23may22.docx

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23 Jun 2022

Dear Dr. Muraca,

Thank you very much for submitting your revised manuscript "Crude and adjusted comparisons of cesarean delivery rates using the Robson classification: A population-based cohort study in Canada and Sweden, 2004-2016" (PMEDICINE-D-22-00231R2) for consideration at PLOS Medicine.

Your paper was evaluated by a senior editor and discussed among all the editors here. It was also discussed with an academic editor with relevant expertise, and sent to three of the original reviewers, including a statistical reviewer. The reviews are appended at the bottom of this email and any accompanying reviewer attachments can be seen via the link below:

[LINK]

In light of the remaining reviewer comments, I am afraid that we will not be able to accept the manuscript for publication in the journal in its current form. However we would like to consider another revised version that addresses the reviewers' and editors' comments. Obviously we cannot make any decision about publication until we have seen the revised manuscript and your response, and we may seek re-review by one or more of the reviewers.

In revising the manuscript for further consideration, your revisions should address the specific points made by each reviewer and the editors. Please also check the guidelines for revised papers at http://journals.plos.org/plosmedicine/s/revising-your-manuscript for any that apply to your paper. In your rebuttal letter you should indicate your response to the reviewers' and editors' comments, the changes you have made in the manuscript, and include either an excerpt of the revised text or the location (eg: page and line number) where each change can be found. Please submit a clean version of the paper as the main article file; a version with changes marked should be uploaded as a marked up manuscript.

In addition, we request that you upload any figures associated with your paper as individual TIF or EPS files with 300dpi resolution at resubmission; please read our figure guidelines for more information on our requirements: http://journals.plos.org/plosmedicine/s/figures. While revising your submission, please upload your figure files to the PACE digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at PLOSMedicine@plos.org.

We expect to receive your revised manuscript by Jun 30 2022 11:59PM. Please email us (plosmedicine@plos.org) if you have any questions or concerns.

***Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.***

We ask every co-author listed on the manuscript to fill in a contributing author statement, making sure to declare all competing interests. If any of the co-authors have not filled in the statement, we will remind them to do so when the paper is revised. If all statements are not completed in a timely fashion this could hold up the re-review process. If new competing interests are declared later in the revision process, this may also hold up the submission. Should there be a problem getting one of your co-authors to fill in a statement we will be in contact. YOU MUST NOT ADD OR REMOVE AUTHORS UNLESS YOU HAVE ALERTED THE EDITOR HANDLING THE MANUSCRIPT TO THE CHANGE AND THEY SPECIFICALLY HAVE AGREED TO IT. You can see our competing interests policy here: http://journals.plos.org/plosmedicine/s/competing-interests.

Please use the following link to submit the revised manuscript:

https://www.editorialmanager.com/pmedicine/

Your article can be found in the "Submissions Needing Revision" folder.

To enhance the reproducibility of your results, we recommend that you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option to publish peer-reviewed clinical study protocols. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

Please ensure that the paper adheres to the PLOS Data Availability Policy (see http://journals.plos.org/plosmedicine/s/data-availability), which requires that all data underlying the study's findings be provided in a repository or as Supporting Information. For data residing with a third party, authors are required to provide instructions with contact information for obtaining the data. PLOS journals do not allow statements supported by "data not shown" or "unpublished results." For such statements, authors must provide supporting data or cite public sources that include it.

We look forward to receiving your revised manuscript.

Sincerely,

Caitlin Moyer

Associate Editor

PLOS Medicine

on behalf of

Louise Gaynor-Brook

Associate Editor

PLOS Medicine

-----------------------------------------------------------

Requests from the editors:

1. Response to reviewers: Please completely address the remaining comments of Reviewer 1 and Reviewer 5. Please carefully consider and respond to the comments provided by Reviewer 5 below.

2. Abstract: Methods and Findings: Line 45: Please revise the sentence for grammar.

3. Abstract: Methods and Findings: Line 45-46: Please separately mention numbers of deliveries for Sweden and BC.

4. Abstract: Line 54: Please use “women with overweight/obesity” in this sentence.

5. Line 55-56: Please also provide the p for trend for changes over time in each country.

6. Abstract: Line 75-76: We suggest revising to “These findings suggest that incorporating these factors into explanatory models may be useful for evaluating cesarean delivery rates in populations.” or similar.

7. Abstract: Conclusions: Please provide 1 sentence of additional interpretation of the findings (e.g. what is the significance of the fact that adjustment for these factors explains different proportions of variation in cesarean delivery between Robson groups? What is the significance that temporal changes in rates can be explained by adjustment for some groups/for Canada?).

8. Author summary: Line 98-99: Please clarify this, as the results do not indicate changes in CD rates over time in Sweden.

9. Author summary: Line 101-102: Please be sure that the author summary points do not reproduce what is written in the Abstract.

10. Methods: Please specify the significance level used (e.g., P<0.05, two-sided) and the statistical tests used to derive p values reported.

11. Results: Please provide both 95% CIs and p values for all applicable results reported in the text.

12. Discussion: Line 374: Please clarify if this is 5.2% of the 8.6% difference in CD rate for group 2 between Canada and BC.

13. Discussion: Line 380: “...but had little impact on temporal changes in CD rates in Sweden.” This is somewhat confusing as the results did not indicate that there were significant temporal changes in CD rates in Sweden. Please clarify.

14. Discussion: Line 381: Please clarify if “industrialized settings” might be more accurately described as high income settings, or similar.

15. References: Please include punctuation after journal title abbreviations. Please remove the registered trademark symbol from reference 30. Please use the "Vancouver" style for reference formatting, and see our website for other reference guidelines https://journals.plos.org/plosmedicine/s/submission-guidelines#loc-references

16. Tables and Supporting Information tables: Please quantify the presentation of results with both 95% CIs and p values where applicable. When reporting p values, please use 2 decimal places for p = 0.01 or larger, and 3 decimal places if smaller. Please report as p<0.001 where applicable. Please check that all abbreviations are defined in legends (e.g. CD).

17. Supporting Information Tables and Figures: Please provide a legend for each table/figure.

18. S1 Table: Please use the "Vancouver" style for reference formatting, and see our website for other reference guidelines https://journals.plos.org/plosmedicine/s/submission-guidelines#loc-references

19. S20 Table: Please clarify the title of the table, as it appears that all years (2008-2016) are being compared to 2004-2007. Please also include p values.

Comments from the reviewers:

Reviewer #1: Thanks for the revised manuscript and responses to my original queries. Your explanation to my question about variation in CD rates was extremely enlightening and I can appreciate why this is not an easy question to answer.

The rest of the changes to the manuscript cover my initial questions (apart from a few minor points below) and I think you have given a fair assessment of the limitations with the missing BMI values. Apart the BMI the amount of missing data is a fairly small fraction and it would take a very unusual missingness mechanism to affect the main results of the study.

If using the 'sequential' term throughout the manuscript I would provide the explanation about what exactly you did (i.e. fitting a series of models with more covariates that could mediate the relationships of interest) to the methods so it's clear for readers. There are some statistical estimation methods that use this term and so best to make sure there's no confusion.

For Table S3 could you just add an brief explanatory note about where there was no p-value for some of the time-series (i.e. 2b, where it stayed the same across time)?

For Tables S5-S16, I would suggest the term 'unbalanced' instead of 'significant' as the convention is for 'significant' to be used with null hypothesis statistical testing.

Reviewer #2: Happy with these responses. I am impressed at the thoroughness of the response to reviewer letter, many thanks.

Reviewer #5: I do not disagree with your conclusion that you need other data in addition to the TGCS to explain differences in the caesarean section. Everybody would agree with that even before your study

I do however disagree that the criteria you have selected prove that or indeed we should encourage clinicians to collect those criteria before more relevant intrapartum data is collected. (i find it extraordinary that you cannot get information on indications, oxytocin rates or fetal outcome)

As a clinician i would have liked to like to see in your paper the TGCS as you have presented and within each of the groups a detailed analysis of relevant events and outcomes including epidemiological data. I also would expect to see in a paper about differences in cs rates a comparison of the indications and also other fetal and maternal outcomes. That way i would be able to compare my own data with yours.

There should also be some description of the clinical processes and how they differ in the two regions.

I also advise caution using 2a as a separate group without 2b. in your dataset it may make no difference but in datasets where group 2b is relatively bigger you may draw inappropriate conclusions. It is not a Robson group and i would be happy to explain the reasons why that may be a problem

I really do believe that your paper shows the limitations of routine perinatal information at the present time and that should be the correct message from your paper nothing else

Any attachments provided with reviews can be seen via the following link:

[LINK]

29 Jun 2022

Submitted filename: R2. Response to Reviewers 28jun22.docx

Click here for additional data file.

13 Jul 2022

Dear Dr. Muraca,

Thank you very much for re-submitting your manuscript "Crude and adjusted comparisons of cesarean delivery rates using the Robson classification: A population-based cohort study in Canada and Sweden, 2004-2016" (PMEDICINE-D-22-00231R3) for review by PLOS Medicine.

I have discussed the paper with my colleagues and the academic editor. I am pleased to say that provided the remaining editorial and production issues are dealt with we are planning to accept the paper for publication in the journal.

The remaining issues that need to be addressed are listed at the end of this email. Any accompanying reviewer attachments can be seen via the link below. Please take these into account before resubmitting your manuscript:

[LINK]

***Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.***

In revising the manuscript for further consideration here, please ensure you address the specific points made by each reviewer and the editors. In your rebuttal letter you should indicate your response to the reviewers' and editors' comments and the changes you have made in the manuscript. Please submit a clean version of the paper as the main article file. A version with changes marked must also be uploaded as a marked up manuscript file.

Please also check the guidelines for revised papers at http://journals.plos.org/plosmedicine/s/revising-your-manuscript for any that apply to your paper. If you haven't already, we ask that you provide a short, non-technical Author Summary of your research to make findings accessible to a wide audience that includes both scientists and non-scientists. The Author Summary should immediately follow the Abstract in your revised manuscript. This text is subject to editorial change and should be distinct from the scientific abstract.

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We ask every co-author listed on the manuscript to fill in a contributing author statement. If any of the co-authors have not filled in the statement, we will remind them to do so when the paper is revised. If all statements are not completed in a timely fashion this could hold up the re-review process. Should there be a problem getting one of your co-authors to fill in a statement we will be in contact. YOU MUST NOT ADD OR REMOVE AUTHORS UNLESS YOU HAVE ALERTED THE EDITOR HANDLING THE MANUSCRIPT TO THE CHANGE AND THEY SPECIFICALLY HAVE AGREED TO IT.

Please ensure that the paper adheres to the PLOS Data Availability Policy (see http://journals.plos.org/plosmedicine/s/data-availability), which requires that all data underlying the study's findings be provided in a repository or as Supporting Information. For data residing with a third party, authors are required to provide instructions with contact information for obtaining the data. PLOS journals do not allow statements supported by "data not shown" or "unpublished results." For such statements, authors must provide supporting data or cite public sources that include it.

To enhance the reproducibility of your results, we recommend that you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option to publish peer-reviewed clinical study protocols. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript.

Please note, when your manuscript is accepted, an uncorrected proof of your manuscript will be published online ahead of the final version, unless you've already opted out via the online submission form. If, for any reason, you do not want an earlier version of your manuscript published online or are unsure if you have already indicated as such, please let the journal staff know immediately at plosmedicine@plos.org.

If you have any questions in the meantime, please contact me or the journal staff on plosmedicine@plos.org.

We look forward to receiving the revised manuscript by Jul 20 2022 11:59PM.

Sincerely,

Caitlin Moyer, PhD

Associate Editor

PLOS Medicine

plosmedicine@plos.org

------------------------------------------------------------

Requests from Editors:

1. Title Page: Please remove the funding and conflict of interest disclosure statements from the main text, and please be sure all information is submitted completely and accurately in the appropriate sections of the manuscript submission system.

2. Abstract: Methods and Findings: Line 59-63: Please also provide p values in addition to the 95% CIs reported for the main results comparing BC to Sweden.

3. Methods: Line 153: We suggest “data extraction” rather than “data manipulation” or similar.

4. Methods: Line 183-184: Please revise to “Temporal trends in the relative contribution of each Robson Group to the overall CD rate were also described.” if this was intended.

5. Results: Line 269: Please revise to “...while women of advanced maternal age (≥35 years) and women with overweight or obesity (≥25 kg/m2) constituted 23.5% and 32.4% of the study population…” or similar.

6. References: For reference 11, please change the journal to PLoS One. Please check that all references use the "Vancouver" style for reference formatting, and see our website for other reference guidelines: https://journals.plos.org/plosmedicine/s/submission-guidelines#locreferences

7. Page 29: Please remove the author contribution, declaration of interests, and data sharing sections from the main text. Please be sure all information is entered completely and accurately into the relevant sections of the manuscript submission system.

8. S20 Table and S24 Table: Please clarify “sequential adjustment” in the table legends along the lines of Reviewer 1’s point: ““The sequential approach was carried out by fitting a series of models with additional groups of factors added to each model in the sequence outlined above to quantify the contribution of each group of factors to CD trends over time.”

9. Figure 4: Please revise the y axis scales consistent across all panels, beginning with zero. If not feasible to do this, please show a break in the axis / include a note in the legend to call attention to the fact that the y axis scales differ for the presentations of Robson Groups 1, 2a, and 5.

10. STROBE Checklist: For item 22, please indicate the section as Funding or similar.

11. S1 Figure: Please include a legend. Please be sure to define what the error bars represent, and the abbreviation RG.

12. S2 Table: Please clarify if these are crude or adjusted OR. If applicable, please present both adjusted and unadjusted results. In the legend, please mention the factors adjusted for, if adjusted analyses were done.

Any attachments provided with reviews can be seen via the following link:

[LINK]

13 Jul 2022

Submitted filename: R3. Response to Editors_13jul22.docx

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15 Jul 2022

Dear Dr Muraca,

On behalf of my colleagues and the Academic Editor, Gordon C. Smith, I am pleased to inform you that we have agreed to publish your manuscript "Crude and adjusted comparisons of cesarean delivery rates using the Robson classification: A population-based cohort study in Canada and Sweden, 2004-2016" (PMEDICINE-D-22-00231R4) in PLOS Medicine.

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Please also address the following editorial request:

-Page 1: Paper presentation information: Please remove, or include as part of the acknowledgements.

PRESS

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Thank you again for submitting to PLOS Medicine. We look forward to publishing your paper.

Sincerely,

Caitlin Moyer, Ph.D.

Associate Editor

PLOS Medicine