Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical Centre in Northern Tanzania: A registry based cohort study.

BACKGROUND: Preterm birth is a public health problem particularly in low- and middle-income countries especially in sub-Saharan Africa. It is associated with infant morbidity and mortality. Survivor of preterm suffers long term health consequences such as respiratory, hearing and visual problems as well as delivering preterm infants. Preterm birth also tends to recur in subsequent pregnancies. Little is known about recurrent rate of preterm birth and associated factors in Tanzania. This study aimed to determine the recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical Centre (KCMC), in Northern Tanzania.
METHODS: A historic cohort study was designed using maternally-linked data from KCMC medical birth registry. Women who delivered 2 or more singletons were included. A total of 5,946 deliveries were analysed. Recurrence of preterm birth and associated risk factors were estimated using multivariable log-binomial regression model with robust standard error to account for repeated births from the same mother.
RESULTS: Overall recurrent rate of preterm birth was 24.4%. The recurrence of early preterm birth was higher compared to late preterm birth (26.2% vs. 24.2%). Similar pattern of recurrence was observed for spontaneous and medically indicated preterm birth (13.5% vs. 10.9%, respectively). Previous preterm birth (RR;1.85, 95% CI: 1.49, 2.31), preeclampsia (RR;1.46, 95% CI: 1.07, 2.00), long inter-pregnancy interval (RR;1.22, 95% CI: 1.01, 1.49) and clinical subtypes (RR = 1.37, 95% CI: 1.00, 1.86) were important predictors for recurrent preterm birth.
CONCLUSION: Recurrence of preterm birth remains higher in this population. The rate of preterm recurrence was dependent of gestational age and sub-clinical subtype. Other factors which were associated with recurrence of preterm birth were previous preterm birth, preeclampsia and long inter-pregnancy interval. Early identification of high risk women during prenatal period is warranted.

Introduction

Globally, approximately 15 million babies are born preterm annually [1]. Majority (>60%) of preterm births occur in low and mid-income countries particularly in South Asia and sub-Saharan Africa [2]. More than 1 million babies who are born preterm die annually due to preterm related complications while those who survive suffers long-term lifetime consequences such as learning disability, visual and hearing problems [3]. Furthermore, preterm birth (PTB) is a leading cause of neonatal morbidity and mortality [4, 5]. It accounts for 35% of neonatal mortality globally each year [4]. The severity of PTB differs across the gestational age [6]. A systematic analysis from 171 countries in low and high income countries reported the PTB rate ranging from 5% to 18% [4]. In Tanzania, approximately 2 million births occur annually, where 11% of these babies are born preterm [7]. In Tanzania, PTB has been reported to account for 10% of under five mortality [7].

Previous investigators have demonstrated that women who had previous history of PTB are at higher risk of experiencing PTB in their subsequent pregnancies [8]. The authors noted that the recurrence can be as high as 30% [8]. Similarly, a hospital-based study in Tanzania reported PTB recurrence rate of 17%, in this study the recurrence of PTB accounted for 15% of perinatal mortality [9]. The recurrence of preterm birth is dependent on the gestational age of the index pregnancy, it also vary by clinical subtype. The shorter gestation age in the index pregnancy has also been reported to increase the risk of PTB in the subsequent pregnancy [10]. Previous investigators have showed that 70% of preterm births occur spontaneously [8] while 30% are medically indicated preterm birth [11].

Numerous factors have been associated with an increased risk of PTB and its recurrence. These include history of preterm birth, low maternal body mass index, pre-eclampsia, placenta previa, and short cervical length [12, 13]. Furthermore, genital infection, multiple gestation, short interpregnancy interval, uterine anomalies, smoking, black race, raised fetal fibronection concentration, psychological stress, stressful working condition, poor living condition, low level of education and maternal unemployment status have aso been associated with an increased risk of PTB and its recurrence [14-16].

Previous studies in the developed countries have reported that the recurrence risk of PTB varies according to gestational age of the previous pregnancy [6]. The authors noted that recurrent PTB also affect the survival of preterm in the successive pregnancy. However, a previous study in Tanzania using the same data used by the present study did not assess the effect of gestational age category of the index pregnancy and its implications on recurrent preterm birth and perinatal outcomes (Mahande et al., 2013). This might have underestimated the rate of PTB recurrence and associated perinatal outcomes. Furthermore, some authors have also reported that the recurrence risk of PTB varies by clinical-subtype [11, 17]. This also have not been explored in resource setting including Tanzania. This study aimed to determine the recurrence rate of PTB across gestational age of the index pregnancy and according to clinical subtypes among women who delivered at Kilimanjaro Christian Medical Center. The study also explored the factors associated with recurrence of PTB across gestational age and by clinical subtype.

Understanding the recurrence rate of PTB is critical as it provides information to clinicians to guide clinical counselling for women at risk of recurrent preterm birth. Information on risk factors for recurrence of PTB are important to researchers as it suggest the etiological factors for PTB and thus guide development of focused interventions to prevent recurrent PTB and its associated consequences. Averting the perinatal mortality attributed by PTB and its complications may help to accelerate the achievement towards Sustainable Development Goal 3.2 which aims to end preventable deaths of new-borns and children under 5 years, with all countries targeting to reduce neonatal mortality to at least as low as 12/1000 live births and under 5 mortality to at least as low as 25/1000 live births by 2030.

Methods

Study setting and design

This was a hospital-based retrospective cohort study which was designed using maternally-linked data from KCMC Medical Birth Registry for women with consecutive deliveries between 2000 and 2015. The KCMC hospital is located in Moshi, northern Tanzania. KCMC is one of four referral hospital in Tanzania serving more than 1.64 million people [18]. Approximately 4,000 deliveries are recorded annually. The Obstetrics and Gynaecology Department at KCMC receives 80% of patients from Moshi who are self-referred which is the main catchment area, while 20% of patients are referred from other nearby regions. KCMC Medical Birth Registry was established in 2000 by University of Bergen Norway in collaboration with Kilimanjaro Christian Medical University College.

Study population and sample size

We included all women who delivered singleton for least two consecutive births at KCMC between 2000 and 2015. All births with missing gestational age, gestational age less than 28 weeks gestational age, multiple pregnancies and those who were referred from rural due to medical complications were excluded. The final sample size comprised of 5,946 consecutive singleton deliveries.

Data collection

The birth registry recorded information for all mothers who delivered in the obstetrics and gynaecology department at KCMC. Trained midwives carry out daily interviews using a standardized questionnaire within 24 hours of delivery, or as soon as a mother has recovered. Other relevant information was obtained from antenatal cards and medical records. All data were stored in a computerized database system at the medical birth registry office.

Variable definitions

The outcome of interest was recurrence of preterm birth. The recurrent preterm birth was defined as two or more deliveries of live babies before 37 completed weeks of gestation [8, 19]. The preterm birth was categorized based on gestation age (early or late PTB). Early preterm birth was defined as a birth of a live baby between 28 and < 32 weeks of gestation age while late preterm birth was referred to birth of a live baby between 32 and <37 weeks of gestation age. Furthermore, the preterm birth was also categorized according to clinical subtypes (i.e. occurred spontaneously or due to medical indication).

The primary exposure of interest preterm birth in the index pregnancy. Other covariates included birth weight, pre-eclampsia, maternal age, maternal education level, residence, occupation, alcohol use in pregnancy, clinical subtypes, inter-pregnancy interval (IPI) and premature rupture of membranes in the index pregnancy. Inter-pregnancy interval was recorded as continuous then categorized into normal IPI (24 to 59 months), short IPI (<24 months) and long IPI (>59 months).

Data analysis

Data cleaning and analysis was done using STATA version 13.1 (Stata Corp, College Station, TX, USA). Continuous variables were summarized using measures of central tendency with their respective measure of dispersion while categorical variables were summarized using frequencies and percentages. Using births as unit of analysis, multivariable log-binomial regression model with robust standard error was used to estimate the recurrence rate of PTB with their 95% confidence intervals while considering the repeated observations (births) from the same mother. All variables which were significant at 20% in bivariate analysis were included in multivariable analysis. In multivariable model, previous preterm was treated as the main exposure of interest. The risk ratios was calculated to find the association of the outcome of interest and the exposures under study. Akaike information criteria (AIC) was used to select the best model in a backwards elimination. A P-value of less than 0.05 was considered significance.

Ethical considerations

The study was approved by Kilimanjaro Christian Medical University College Research and Ethics Committee prior to commencement of the study. The informed verbal consents were obtained from each participant before recruitment. For women who did not consent to participate in the study were not included. The confidentiality and privacy were assured by the use of mothers’ identification numbers and private rooms for interviews.

Results

Socio-demographic characteristics of the study participants

A total of 5,946 consecutive singleton deliveries were analysed (Fig 1). The mean age of the mother’s was 30.4 (SD = 5.1) years. Majority (64.7%) were aged between 25–35 years. More than a third (68.3%) were from urban and. more than a quarter (29.4%) reported using alcohol during pregnancy (Table 1).

Fig 1

Flow chart of the number of deliveries, inclusions and exclusions during the study period.

Table 1

Socio-demographic characteristics of women with consecutive deliveries (N = 5,946).

Variables	n	%
Mother’s age (years)
15-<25	786	13.2
25–35	3,848	64.7
>35	1,303	21.9
Missing	9	0.2
Mother’s residence
Rural	1,640	27.6
Urban	4,304	68.3
Missing	2	0
Marital status
Not married	185	3.1
Married	5,761	96.9
Educational level of mother
None	56	0.9
Primary	2,880	48.4
Secondary	872	14.7
Tertiary	2,138	36
Mother’s occupation
Employed	4,486	75.4
Unemployed	1,445	24.3
Missing	15	0.3
Alcohol use
No	4,196	70.6
Yes	1,750	29.4
Religion
Catholic	2,526	42.5
Protestant	2,385	40.1
Muslim	1,008	17
Others	17	0.3
Missing	10	0.2
Mother’s tribe
Chagga	3,864	65
Pare	720	12.1
Others	1,362	22.9

Clinical characteristics of the study participants

Only 182 (3.1%) reported to have pre-eclampsia and 727 (12.2%) had malaria during pregnancy. Majority (87.8%) of women had normal birth weight. Sixty eight percent of women had spontaneous vaginal delivery (68.15%) while the remaining had medically indicated deliveries. Nearly half (47.2%) of the women had optimal inter pregnancy interval. The majority (85.7%) had a parity of two to five. A third (63.1%) of the women had more than 4 antenatal care visits (Table 2).

Table 2

Clinical characteristics of mothers with consecutive deliveries at KCMC (N = 5,946).

Variables	n	%
Pre-eclampsia
No	5,764	96.9
Yes	182	3.1
Birth weight
Normal birth weight	5,542	93.2
Low birth weight	359	6
Very low birth weight	37	0.6
Extremely low birth weight	8	0.1
Malaria during pregnancy
No	5,219	87.8
Yes	727	12.2
Clinical subtypes
Spontaneous delivery	4,052	68.15
Medically indicated delivery	1,884	31.69
Missing	10	0.17
Inter-pregnancy interval (months)
24–59	2,805	47.2
<24	1,902	32
>59	1,049	17.6
Missing	190	3.2
Parity
Prime	734	12.3
Multiparity	5,093	85.7
Grand multiparity	119	2
Antenatal care attendance
4 and more visits	3,751	63.1
Less than 4 visits	2,139	36
Missing	56	0.9

Recurrence of preterm birth

The overall rate of recurrence of preterm birth in this population was 24.4%. The rate of recurrence of early preterm birth was slightly higher compared to late preterm birth (26.2% vs. 24.2%, respectively). In the early preterm birth category, 20% of deliveries occurred spontaneously while 2.2% were medically indicated. In late preterm birth category, 16.6% of deliveries occurred spontaneously while 5.1% were medically indicated (Table 3).

Table 3

Classification of rate of preterm birth recurrence by clinical subtypes and gestational age categories (N = 5946).

			Current preterm birth categories
	Total	Term	Preterm	sPTB	mPTB
	N	n (%)	n (%)	n (%)	n (%)
Previous Preterm deliveries
Term delivery	5,405	4880 (90.3)	525 (9.7)	330 (6.1)	195 (3.6)
Preterm delivery	541	409 (75.6)	132 (24.4)	73 (13.5)	59 (10.9)
Spontaneous delivery	4,309	3863 (89.6)	446 (10.4)	358 (8.3)	88 (2.1)
Medically indicated delivery	1,619	1408 (87.9)	211 (13.1)	45 (2.8)	166 (10.3)
Early preterm birth	61	45 (73.8)	16 (26.2)	10 (16.4)	6 (9.8)
Spontaneous delivery	45	35 (77.8)	10 (22.2)	9 (20)	1 (2.2)
Medically indicated delivery	16	10 (62.5)	6 (37.6)	1 (6.3)	5 (31.3)
Late preterm birth	480	364 (75.8)	116 (24.2)	63 (13.1)	53 (11)
Spontaneous delivery	314	246 (78.3)	68 (21.7)	52 (16.6)	16 (5.1)
Medically indicated delivery	163	115 (70.6)	48 (29.4)	11 (6.7)	37 (22.7)

On the other hand, the recurrence rate of spontaneous preterm birth was higher compared to medically indicated preterm birth (13.5% vs 10.9%, respectively). The rate of spontaneous preterm birth in previous preterm which recurred spontaneously was 8.3%. The rate of spontaneous preterm birth in prior pregnancy which recurred due medical indication was 2.1%. The rate of medically indicated preterm birth which recurred spontaneously was 2.8% while the rate of medically indicated preterm birth in prior pregnancy which recurred due to medical indication was 10.3% (Table 3).

The rate of early medically indicated preterm birth in previous pregnancy which recurred spontaneously was 6.3% while 31.3% recurred due to medical indication. The rate of late medically indicated preterm birth which recurred spontaneously was 6.7% while 22.7% recurred due to medical indications (Table 3).

In bivariate analysis (Table 4), the factors associated with recurrent preterm birth which were statistically significant at 20% level were previous preterm birth, pre-eclampsia, inter-pregnancy interval, birth weight, clinical subtypes and alcohol use. After adjusting for mother’s age, mother’s education, premature rupture of membrane and alcohol use, factors which were statistically significant were previous preterm birth, pre-eclampsia, inter-pregnancy interval and clinical subtypes.

Table 4

Factors associated with recurrent preterm births among women delivered at KCMC (N = 5946).

Characteristics		Recurrence of Preterm birth
	Total (N)	n (%)	cRR,95% CI	P-value
in previous birth)
Term delivery	5,405	525 (9.7)	1
Preterm delivery	541	132(24.4)	2.51[2.11,2.97]	<0.001
Pre-eclampsia
No	5,763	622(10.8)	1
Yes	183	35(19.1)	1.77[1.29,2.41]	<0.001
Inter-pregnancy interval (months)
24–59	2,805	281(10)	1
<24	1,902	227(11.9)	1.19[1.01,1.40]	0.036
>59	1,049	130(12.4)	1.24[1.02,1.51]	0.034
Birth weight
Normal birth weight	5,441	538(9.9)	1
Low birth weight	442	98(22.2)	2.24[1.85,2.71]	<0.001
Very low birth weight	53	19(35.8)	3.62[2.51,5.21]	0.001
Extremely low birth weight	10	2(20)	2.02[0.58,6.99]	0.267
Premature rupture of membrane
No	5,873	642(10.9)	1
Yes	73	15(20.5)	1.00[0.57,1.76]	0.988
Clinical subtypes
Spontaneous delivery	359	78(21.7)	1
Medically indicated delivery	178	54(30.3)	1.40[1.03,1.88]	0.028
Term delivery	5409	527(9.74)	0.45[0.36,0.57]	<0.001
Mother’s education
No formal education	56	7(12.5)	1
Primary education	2,880	350(12.2)	0.98[0.49,1.96]	0.945
Secondary education	872	105(12)	0.96[0.47,1.96]	0.906
Tertiary education	2,138	195(9.1)	0.73[0.36,1.48]	0.384
Alcohol use
No	4,196	485(11.6)	1
Yes	1,750	172(9.8)	0.85[0.72,1.00]	0.055
Mother’s age (years)
15-<25	786	100(12.7)	1
25–35	3,848	404(10.5)	0.82[0.67,1.01]	0.059
>35	1,303	152(11.7)	0.92[0.72,1.16]	0.46

cRR: Crude risk ratio

The bivariate analysis for the factors associated with recurrent preterm birth has been shown in Table 5. Women who had previous preterm birth had nearly 2-fold risk of preterm birth in their subsequent pregnancies compared to women who had term pregnancies in the previous pregnancies (RR = 1.85; 95% CI: 1.49-, 2.31). Women who had preeclampsia in previous pregnancies had 50% (RR = 1.46; 95% CI: 1.07, 2.00) higher risk of preterm birth in their subsequent pregnancies as compared to women who had normotensive pregnancy in their previous pregnancy. Women who had short IPI in the previous pregnancy had 1.13 times (95% CI: 0.96, 1.33) higher risk of having future preterm birth compared to women who had optimal IPI whereas women who had long IPI had 22% (RR = 1.22, 95% CI: 1.01, 1.49) higher risk of having preterm birth in their future pregnancies as compared to their counterparts who had optimal IPI.

Table 5

Factors for recurrence of preterm birth delivery by women characteristics in previous pregnancy (N = 5,946).

Variables	cRR,95% CI	aRR,95% CI	P-value
Birth type
Term delivery	1	1
Preterm delivery	2.51[2.11,2.97]	1.85[1.49,2.31]	<0.001
Pre-eclampsia
No	1	1
Yes	1.77[1.29,2.41]	1.46[1.07,2.00]	0.017
Inter-pregnancy interval
Normal IPI	1	1
Short IPI	1.19[1.01,1.40]	1.13[0.96,1.33]	0.148
Long IPI	1.24[1.02,1.51]	1.22[1.01,1.49]	0.042
Clinical subtypes
Spontaneous delivery	1	1
Medically indicated	1.40[1.03,1.88]	1.37[1.00,1.86]	0.048
Term delivery	0.45[0.36,0.57]	0.59[0.46,0.77]	<0.001

cRR: Crude risk ratio, aRR: Adjusted risk ratio

Women who delivered by medical indication had 37% (RR = 1.37, 95% CI: 1.00, 1.86) higher risk of delivering preterm birth in their subsequent pregnancies compared to those who had spontaneous preterm deliveries. Furthermore, women who had term deliveries had 41% lower risk of having preterm deliveries in their subsequent deliveries compared to women who delivered spontaneously (RR = 0.59, 95% CI: 0.46, 0.77). Women whose babies had low birth weight in their previous pregnancy had 1.57 times (95% CI: 1.25, 1.99) higher risk of delivering preterm babies in the subsequent pregnancies compared to women who had babies with normal birth weight. Women whose babies had very low birth weight had 2 times (95% CI: 1.29, 3.10) higher risk of preterm birth recurrence compared to women whose babies had normal birth weight. In the other hand, women whose babies had extremely low birth weight had 1.13 times (95% CI: 0.31, 4.10) higher risk of preterm birth recurrence compared to those with babies having normal birth weight.

Discussion

This study aimed to determine the recurrence of preterm birth and it associated factors among women with subsequent births. The overall recurrence rate of preterm birth was 24.4%. The recurrence of early preterm birth was slightly higher compared to the late preterm birth (26.4% vs. 24.2%, respectively). Similarly, the recurrence of spontaneous preterm birth was higher compared to medically indicated preterm birth (13.5% vs. 10.9%, respectively). The previous preterm birth, inter-pregnancy interval, preeclampsia were significantly associated with recurrence of preterm birth.

The rate of preterm birth recurrence of 24.4% observed in the present study is higher than the recurrence rate of 17% which was previously reported in 2008 using the same dataset [9]. This reflect that the rate of preterm birth recurrence has increased by 43.5% percentage points over the last seven years. This increase may due increased medical induced deliveries. This calls for a need for concerted effort to address the risk factors associated with preterm birth in order to reduce this rising recurrence of preterm birth. The previous study in Canada reported a preterm birth recurrence rate of 30% [8] which is higher compared to the findings of this study. These differences may be explained by the difference in study designs between the two studies. The former was a meta-analysis, which reported over 30 clinical trials and cohort findings. Probably this might have more precision and high quality data than the present study. Another possible explanation could be that the recurrence rate of preterm birth is higher in low resourced countries than in high resource settings, but it looks lower due to poor data quality and a huge number of unreported cases. However, despite of having slightly lower rate of recurrence, the survival gap for preterm babies is high between the two settings with majority (90%) of all preterm babies die in low resourced settings as compared to their counterparts born in developed world where only 10% of preterm babies die [1].

We found a differential in preterm birth recurrence by gestational age category where 26.2% of recurrence occurred in early preterm. This information is important because babies at this gestational account many challenges in terms of management and care especially in low resource settings due to unavailability of high impact interventions known to serve lives for preterm babies. A study done in Canada reported that gestation age is inversely proportion to increased morbidity and mortality [8]. Therefore, special care is needed for babies in this category especially in our settings where there is still a challenge in caring of early preterm deliveries. 24.2% of late preterm birth recurred at late preterm birth category.

There was a variation in recurrence of preterm birth by clinical subtypes and across gestational categories. The recurrence rate of spontaneous preterm birth was higher than the recurrence of medically indicated (13.5% vs. 10.9%, respectively). However, the recurrence of spontaneous preterm birth in our study was lower compared to 70% that was reported elsewhere [8]. This difference could be explained by the differences in population characteristics between the studies populations. Medically indicated preterm birth have been a reason for the rise of preterm birth in the US whereby it contributes up to 30% of all preterm births [11]. In the presented study, medically indicated preterm births accounted for 44.6% of all recurrent preterm birth in the study setting. These calls for targeted intervention to prevent medically indicated preterm births thereby helping to reduce the overall preterm birth rate and associated cost implications.

Previous preterm birth was the main reason for recurrence of preterm birth in our hospital. Women who had previous preterm birth were at higher risk of recurrence compared to women who had term delivery in their previous pregnancies. These findings were similar to the study done in Canada which reported that previous preterm was the main risk for future preterm birth from the same mother [8]. This calls for a need for early identification of women at risk of recurrence through counselling about the syndrome and for possible management and close follow-up during the prenatal care.

Other factors which were associated with recurrence of preterm birth include medically indicated preterm birth, preeclampsia and long IPI. Medically indicated preterm birth were having higher risk of recurring in subsequent pregnancies compared to spontaneous deliveries. Also, this study found that term deliveries were less likely to have siblings who will be born before term either spontaneously or medically indicated. This is consistent with previous studies [11, 17, 20]. Authors in the previous studies suggested that the clinical subtypes share the etiologies which could explain the observed results.

Women with preeclampsia in previous pregnancy had higher risk preterm birth in their subsequent pregnancies. This finding is in agreement with previous investigators in developed countries [17]. Proper preeclampsia management is vital in averting the recurring preterm birth due to this factor. Long IPI was found to be associated with recurrence of preterm birth. Our finding was in congruent with previous studies in northern Tanzania [21]. This suggests a need for use of family planning methods to ensure optimal IPI thereby averting preterm birth attributed to long IPI. Women should be educated on the risk of long IPI on preterm birth during ANC visits.

Strengths and limitations

The strength of this study is the use of large sample of pregnant women who delivered consecutively at KCMC for the past 15 years. This gave a high power of a study to detect the association of different factors with recurrence of preterm birth. This data accounted for clustering effect with robust variance since consecutive deliveries of the same mothers were studied. To the best of author’s knowledge, this is the first study to describe recurrence of preterm birth by clinical subtypes and by gestational age categories in Tanzania.

However, this study had some limitations which need to be taken into accounting when interpreting our finding. Use of secondary data lead to missing some variables which are risk for preterm birth and possibly its recurrence such as paternal change between successive pregnancies and behavioural factors.

Conclusion

Recurrence of preterm birth is still a major public health problem. The gestation age at delivery is a predictor of the morbidity and mortality due to preterm births recurrence. The factors which were associated with recurrence of preterm birth were previous preterm birth, preeclampsia, long inter-pregnancy interval, and clinical subtypes of preterm birth. Women with history of preterm birth should be identified as high risk group and counselling should be done with regards to the recurrence of preterm birth.

(DOCX)

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6 May 2020

PONE-D-19-30288

Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical centre in Northern Tanzania: a registry based cohort study

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The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: No

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Preterm birth is an important medical problem needing academic efforts. This work was very well done, however, I did not learn anything by reading the manuscript. All the results are as expected and no new discovery would benefit the research and management of preterm birth. It is not challenging academically, therefore, I suggest the rejection of the work.

Reviewer #2: Overall:

It’s an important study as preterm labour and delivery is an important cause of perinatal deaths and long term complications in the newborns. This information will benefit the studied community as the gained information will be used in managing future pregnancies of patients in this locality and Tanzania in general.

General comment:

There are few grammatical errors which need to be corrected.

Title: Well written, clear, and in line with the objectives. It explains what has been researched.

Abstract: The abstract is provided and it contains all the important elements of the study.

Introduction:

The subject has been well researched. There is mixed information about the concept of risk and recurrence in second and third paragraphs, which I feel should be separated.

Last but one paragraph shows that the “study will be done”. This needs to be written in past tense.

Methods:

The methods have been explained, and have provided answers to raised questions, but do not explain whether this was a prospective or retrospective study. This could explain the data that was missing.

The category for the late preterm birth as explained it the first paragraph of variable definitions is “<37 weeks of gestation age, which means 36 completed weeks. This means there are women who were missed who were in the 37th week and had not attained 37 completed weeks

Ethical considerations:

It was ethical to carry out this study as it caused no reported harm to the participants.

Ethical issues were well treated as women gave informed consent, even though it is not explained whether there were women who did not give consent and what happened to them.

Results:

The results section is provided, and explains the results well.

There is too much text that explains the results while the tables are provided. The narrative part can concentrate on highlighting only important findings in the table.

Some sentences are not very clear, and few are hanging.

Discussion:

This is provided and has discussed the results of their study but it does not discuss very well the results and link them well with other previous studies.

Conclusion:

Conclusions have been given. It’s important to limit the conclusions to only what has been studied.

Reviewer #3: Thank you for the opportunity to review this manuscript of an important topic in low income country setting.

Here are my comments to improve the manuscript.

ABSTRACT:

The conclusion misses some of the important variables that were significant Need to mention the numerous factors associated with the recurrence.

INTRODUCTION

Please put the long form of KCMC on page 4 paragraph 1 because it is used for the first time.

Write Sustainable Development Goals and not sustainable development goals on page 4 paragraph 2

METHODOLOGY

Study population and study sample: It is good practice to show power calculation to see whether the sample size in the database is enough to depict the outcome intended.

Why were the referred cases with medical complications from rural area excluded. Some could have been referred due to conditions that needed medically induced delivery in a tertiary institution.

Variable definitions: There are other important conditions associated with PTb such as diabetes mellitus, chronic hypertension. But these were not factored in. Can you explain if these conditions are not captured in the database.

Data Analysis: You should add a description about risk ratio in the text.

Ethical considerations: Write long form for KCMUCo

RESULTS

Page 6 last paragraph:"Through spontaneous vaginal delivery (68.15%)". This sentence is hanging it is not clear.

Table 1: Please check the age range 15 -25 and 25 -35. It seems 25 years are included into the two groups which is misleading in my opinion

Table2: Please rearrange the IPI starting with <24 then 24-29 etc. On parity how do you have primigravida if the inclusion criteria was two or more singleton deliveries: The number of antenatal visits: what about those with four visits it seems they are missing here.

Table 3: A bit confusing as the row numbers add more than the total. May be you need to remove the column with TOTAL N.

Recurrence of preterm birth ; I would suggest you put a numerator and denominator for there overall recurrent rate of 24.4% as it not very clear from table 3.

DISCUSSION

Page 8 last paragraph:Why do you think it has increased. Is because of referral cases or because of the population increase or because of increase in medically induced deliveries?

Pg 9 Paragraph 1. You can not really make comparison using the meta analysis in my opinion. Rather may be look at the individual papers in meta analysis to make a comparison.

Pg 9 Paragraph 3:For the medically indicated preterm of 44.6%, Probably should say that for medically induced, preterm care should be improved to make sure the premature babies are taken care properly rather than need to be avoided as you can not avoid if delivery is indicated before term.

Paragraph 3 page 10: I think family planning is important but those with long IPI probably were using FP or thee was delay because of other reasons. I think here what is important is to educate the women on the risks of long IPI on preterm birth during the antenatal period.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (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. Registration is free. 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 figures@plos.org. Please note that Supporting Information files do not need this step.

4 Aug 2020

Point by point response to reviewer’s comments

Manuscript ID: Ms. [PONE-D-19-30288R1]

PONE-D-19-30288

Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical centre in Northern Tanzania: a registry based cohort study

Journal: Plose One

Subject: Author's response to reviewer’s comments

Dear Editor,

Thanks very much for the invitation to resubmit our manuscript entitled: “Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical centre in Northern Tanzania: a registry based cohort study”. We appreciate the reviewers’ useful comments and suggestions to improve our manuscript. We were able to address all the comments and suggestions, and therefore we are expecting that the paper can now be accepted in your journal. Please find our responses are indicated in a red colour. A comments requested have been incorporated in the main text.

Dr. Nathaniel Halide Kalengo

Corresponding author

EDITORIAL COMMENTS

When submitting your revision, we need you to address these additional requirements.

1. Please include additional information regarding the survey or questionnaire used in the study and ensure that you have provided sufficient details that others could replicate the analyses. For instance, if you developed a questionnaire as part of this study and it is not under a copyright more restrictive than CC-BY, please include a copy, in both the original language and English, as Supporting Information. If the original language is written in non-Latin characters, for example Amharic, Chinese, or Korean, please use a file format that ensures these characters are visible.

We have included a copy of questionnaire used in this tudy as suggested by the editor.

2. Please amend your current ethics statement to address the following concerns:

a) Did participants provide their written or verbal informed consent to participate in this study?

Participants provided oral informed consent, particularly because the interview was administered just after the woman had given birth.

b) If consent was verbal, please explain i) why written consent was not obtained, ii) how you documented participant consent, and iii) whether the ethics committees/IRB approved this consent procedure.

As indicated above, participant provided oral informed consent because interviews were administered just after the woman had given birth. The local Institutional Review Board of KCMC hospital and the National Ethics Committee in Norway approved these procedures when establishing this birth registry.

3. In your Data Availability statement, you have not specified where the minimal data set underlying the results described in your manuscript can be found. PLOS defines a study's minimal data set as the underlying data used to reach the conclusions drawn in the manuscript and any additional data required to replicate the reported study findings in their entirety. All PLOS journals require that the minimal data set be made fully available. For more information about our data policy, please see http://journals.plos.org/plosone/s/data-availability.

Upon re-submitting your revised manuscript, please upload your study’s minimal underlying data set as either Supporting Information files or to a stable, public repository and include the relevant URLs, DOIs, or accession numbers within your revised cover letter. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories. Any potentially identifying patient information must be fully anonymized.

Important: If there are ethical or legal restrictions to sharing your data publicly, please explain these restrictions in detail. Please see our guidelines for more information on what we consider unacceptable restrictions to publicly sharing data: http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions. Note that it is not acceptable for the authors to be the sole named individuals responsible for ensuring data access.

We will update your Data Availability statement to reflect the information you provide in your cover letter.

The KCMC medical birth registry data contains potentially identifying and sensitive patient information. This has also been stipulated by the local Institutional Review Board of KCMC hospital and the National Ethics Committee in Norway when establishing this birth registry. Permission to use the data in this study was made through the Kilimanjaro Christian Medical University College Research and Ethics Review Committee, and received an approval number 2086. The authors do not have the legal right to share the data publicly. All data requests can be sent directly to the Executive Director of the KCMC referral hospital, P. O. Box 3010, Moshi, Tanzania, Email: kcmcadmin@kcmc.ac.tz or through the corresponding author.

Comments to the Author

Reviewer #2: Overall:

General comment:

There are few grammatical errors which need to be corrected.

We thank the reviewer for important suggestion. We have now corrected all the grammatical errors in the manuscript

Title: Well written, clear, and in line with the objectives. It explains what has been researched.

We do appreciate for the complement.

Abstract: The abstract is provided and it contains all the important elements of the study.

We do appreciate for the complement.

Introduction:

The subject has been well researched. There is mixed information about the concept of risk and recurrence in second and third paragraphs, which I feel should be separated.

Last but one paragraph shows that the “study will be done”. This needs to be written in past tense.

We thank the reviewer for these important observations. We are also very sorry for this confusion. The risk of preterm birth in the subsequent pregnancy and recurrence of preterm birth mean the same in this context.

The phrase ‘study will be done’ has been corrected, as per reviewer suggestion

Methods

The methods have been explained, and have provided answers to raised questions, but do not explain whether this was a prospective or retrospective study. This could explain the data that was missing.

The correction has been done as per reviewer’s comment. This was a retrospective study.

The category for the late preterm birth as explained it the first paragraph of variable definitions is “<37 weeks of gestation age, which means 36 completed weeks. This means there are women who were missed who were in the 37th week and had not attained 37 completed weeks

No women were missed in this categorization since the category < 37 weeks means 36 weeks plus 6 days and not just 36 completed weeks.

Ethical considerations:

It was ethical to carry out this study as it caused no reported harm to the participants.

Ethical issues were well treated as women gave informed consent, even though it is not explained whether there were women who did not give consent and what happened to them.

There were women who did not give consent and were not included in the study, and this will be corrected in the main document as per reviewer’s observation.

Results

The results section is provided, and explains the results well. There is too much text that explains the results while the tables are provided. The narrative part can concentrate on highlighting only important findings in the table. Some sentences are not very clear, and few are hanging.

The results section was paraphrased and some hanging sentences were taken care of as per reviewer’s suggestion.

Discussion

This is provided and has discussed the results of their study but it does not discuss very well the results and link them well with other previous studies.

We understand reviewer’s concern. We have tried to communicate what were the findings and compare them to other studies in the same settings and in different settings in order to come up with relevant solutions in our setting.

Conclusion

Conclusions have been given. It’s important to limit the conclusions to only what has been studied.

This has been taken care of.

Reviewer #3:

Thank you for the opportunity to review this manuscript of an important topic in low income country setting.

Here are my comments to improve the manuscript.

ABSTRACT:

The conclusion misses some of the important variables that were significant Need to mention the numerous factors associated with the recurrence.

The important variables have now been mentioned in this section

INTRODUCTION

Please put the long form of KCMC on page 4 paragraph 1 because it is used for the first time.

Response: Done

Write Sustainable Development Goals and not sustainable development goals on page 4 paragraph 2

Response: Done

METHODOLOGY

Study population and study sample: It is good practice to show power calculation to see whether the sample size in the database is enough to depict the outcome intended.

The convenient sampling method was used and women with more than 1 delivery in the birth registry was included in the study. For a cohort study design with around 5000 participants with more than on point data information is enough number to depict the outcome of interest.

Why were the referred cases with medical complications from rural area excluded. Some could have been referred due to conditions that needed medically induced delivery in a tertiary institution.

The referrals with complications were excluded because they would over estimate the outcome of interest.

Variable definitions: There are other important conditions associated with PTb such as diabetes mellitus, chronic hypertension. But these were not factored in. Can you explain if these conditions are not captured in the database.

These factors are in the database, but they were not statistically significant associated with outcome of interest at crude analysis and were thus factored out.

Data Analysis: You should add a description about risk ratio in the text.

This was added as per reviewer’s suggestion.

Ethical considerations: Write long form for KCMUCo

This has been addressed as per reviewer’s suggestion.

RESULTS

Page 6 last paragraph: "Through spontaneous vaginal delivery (68.15%)". This sentence is hanging it is not clear.

This has been taken care of as per reviewer’s suggestion.

Table 1: Please check the age range 15 -25 and 25 -35. It seems 25 years are included into the two groups which is misleading in my opinion

In the do file analysis the cut off point was 24.999, therefore the greater than or less than signs will be used to clarify this.

Table2: Please rearrange the IPI starting with <24 then 24-29 etc.

Response: We understand the concern of a reviewer, but an IPI of 24 to 59 months is the optimal IPI as per WHO. This was put at the top as it is termed as baseline group, while less than this is short IPI and greater than this is long IPI.

On parity how do you have primigravida if the inclusion criteria was two or more singleton deliveries:

Response: We have primigravida because among deliveries picked with more than 2 records in the database, some of them have their primigravida record, second and third deliveries. i.e. these consecutive deliveries include some first pregnancy to some participants while others may have a record of second to fourth pregnancy.

The number of antenatal visits: what about those with four visits it seems they are missing here.

The correction done as per reviewer’s suggestion, now it read 4 and more visits instead of more than four visits versus less than four.

Table 3: A bit confusing as the row numbers add more than the total. May be you need to remove the column with TOTAL N.

Total N= Term deliveries (5405) in previous pregnancy + preterm deliveries (541) in the previous delivery making a total of 5946. The numbers in the rows may be added up to the term and preterm numbers in the current pregnancy, after that it is the clinical classification of PTB among preterm deliveries in the current pregnancy.

Recurrence of preterm birth; I would suggest you put a numerator and denominator for there overall recurrent rate of 24.4% as it not very clear from table 3.

The numerator for this recurrent rate of 24.4% is preterm deliveries in the current pregnancy (132) while the denominator is preterm delivery in the previous pregnancy (541).

DISCUSSION

Page 8 last paragraph: Why do you think it has increased. Is because of referral cases or because of the population increase or because of increase in medically induced deliveries?

This has been added in the text as per reviewer’s suggestion.

Pg 9 Paragraph 1. You can not really make comparison using the meta analysis in my opinion. Rather may be look at the individual papers in meta analysis to make a comparison.

We understand the reviewer concern. This was the best information available, I think the meta-analysis is a summary of many finding, and to my opinion it is ok to make comparison with the findings from my study.

Pg 9 Paragraph 3:For the medically indicated preterm of 44.6%, Probably should say that for medically induced, preterm care should be improved to make sure the premature babies are taken care properly rather than need to be avoided as you can not avoid if delivery is indicated before term.

Medically indicated preterm birth are those which occur due to medical condition which make us deliver the baby earlier, those are the conditions to be addressed in order to push the gestation age to term or close to term.

Paragraph 3 page 10: I think family planning is important but those with long IPI probably were using FP or thee was delay because of other reasons. I think here what is important is to educate the women on the risks of long IPI on preterm birth during the antenatal period.

This observation is well noted and put to action as per reviewer’s suggestion ________________________________________

Submitted filename: Response to Reviewers comments final.docx

Click here for additional data file.

31 Aug 2020

Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical centre in Northern Tanzania: a registry based cohort study

PONE-D-19-30288R1

Dear Dr. Kalengo,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Joel Msafiri Francis, MD, MS, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

Reviewer #3: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: (No Response)

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: (No Response)

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: (No Response)

Reviewer #3: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: (No Response)

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: (No Response)

Reviewer #3: (No Response)

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

Reviewer #3: Yes: Furaha August

4 Sep 2020

PONE-D-19-30288R1

Recurrence rate of preterm birth and associated factors among women who delivered at Kilimanjaro Christian Medical centre in Northern Tanzania: a registry based cohort study

Dear Dr. Kalengo:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Joel Msafiri Francis

Academic Editor

PLOS ONE