Accuracy of ultrasound estimation of fetal weight at term: A comparison of shepard and hadlock methods.

Background: Ultrasound measurement provides a noninvasive means of obtaining information about fetal weight and may help in necessary preparations at and after delivery. Although some ultrasound methods include only one or two fetal indices, others, to improve accuracy, incorporate either three or all the four fetal indices. The aim of this report is to assess the accuracy of two different methods for fetal weight estimation. Materials and
Methods: This was a prospective study of 170 consecutive pregnant women at term. Ultrasound was used to estimate fetal weight by the Shepard and Hadlock methods, and the actual birth weight (ABW) was determined at birth. The ultrasound-estimated fetal weights (EFWs) and ABW were analyzed.
Results: The women were aged 21-42 years (mean 31.3 ± 7 years). The EFW using the Shepard method was 1.9 kg-5.0 kg (mean 3.6 ± 0.5 kg) and 1.8 kg-4.4 kg (mean 3.3 ± 0.4 kg) for Hadlock method, and ABW was 2.0 kg-4.5 kg (mean 3.4 ± 0.5 kg). The mean EFW using the Shepard method was significantly higher than that of ABW (P < 0.001). The Shepard method significantly overestimated macrosomia compared to that by the ABW. There was no significant difference in microsomia rate between the two methods and ABW.
Conclusion: The Hadlock method was more accurate at estimating fetal weight compared to the Shepard method and is recommended for the ultrasound estimation of fetal weight in our setting and similar settings.

INTRODUCTION

Estimation of fetal weight is important for antenatal and intrapartum clinical decision-making. It is an important component of maternal care, especially in counseling, differential diagnosis, and planning for delivery.[12] Fetal weight estimation is a vital parameter which is not only used in the management of labor and deliveries but also routinely used during antepartum period to evaluate high-risk pregnancies such as diabetic pregnancy, postterm pregnancies, hypertensive disorders in pregnancy, vaginal birth after a previous cesarean delivery, and intrapartum management of fetuses with malpresentation. Management of these clinical conditions is likely to be influenced by the estimated fetal weight (EFW).[3]

Birth weight has been reported to be the single most important parameter that determines neonatal survival.[45] Estimation of fetal weight may contribute to reduction of the high rate of perinatal mortality in low- and middle-income countries, including Nigeria.[4] The burden of extreme fetal weight on maternal and neonatal outcomes has necessitated studies into accurate ways of estimating fetal weight to support appropriate decision-making.[6]

The main method of estimating fetal weight in current obstetrics practice is by ultrasonography. However, it is not clear which ultrasound methods provide more accurate estimation,[7] with regard to one or two fetal biometric parameters versus three or four parameters.[78] The aim of this study was to determine the suitable method (Shepard method with two biometric parameters and Hadlock method with four biometric parameters) which would provide a more accurate method of fetal weight estimation. The hypothesis was that there is no difference in accuracy between Shepard and Hadlock methods in the ultrasound estimation of fetal weight at term.

MATERIALS AND METHODS

This was a prospective study of 170 consecutive pregnant women at term who had reliable pregnancy dating criteria, carried out at the Department of Obstetrics and Gynaecology, University of Abuja Teaching Hospital, Abuja, Nigeria, from April to August 2016.

Study participants

The study participants constituted consecutive pregnant women with singleton term pregnancy in cephalic presentation, admitted in the labor ward for delivery, or scheduled for elective cesarean section. The women had their gestational age confirmed by the last menstrual period (LMP) or ultrasound scanning at 5–20-week gestation. They had been booked for antenatal care and managed according to the existing departmental protocols. Pregnant women with the following conditions were excluded from the study: polyhydramnios, oligohydramnios, preterm labor, premature rupture of fetal membranes, abnormal lie and presentation, multiple pregnancies, antepartum hemorrhage, eclampsia, maternal obesity (absolute weight ≥90 kg), anterior implanted placenta, obvious identified congenital anomaly, and delivery of baby after 48 h of ultrasound estimation of fetal weight (this was to minimize fetal weight changes before delivery). Those in whom fetal parts could not be adequately visualized were also excluded from the study.

The sample size was determined to be 170 using the formula for comparative studies as detailed below: [9]

(where Zcrit = 95%, δ =0.452,[10] the absolute difference in mean value between the two groups aiming to detect significance is determined and the degree of accuracy is set as d = 0.25, value of [Zcrit + Zpwr] 2 at 95% power and 5% level of significance for a two-sided test is = 13.0)

Study protocol

Demographics

Information on age, LMP, gestational age, and parity was obtained. Maternal weight was determined using an adult weighing scale. The mode of delivery was also documented.

Study equipment

The ultrasound machine used was a digital ultrasonic diagnostic system, method Dp3300, two dimensional, real time with an abdominal sector 3.5 MHz transducer.

The adult weighing scale was a standard analog weighing scale (Hana power®). It was used to measure the weights of the participants in kilograms.

The newborn weighing scale was the standard analog waymaster (England) scale corrected for zero error.

Estimation of fetal weight

Ultrasound estimation of fetal weight was performed using the same ultrasound machine for all the study participants, under the supervision of a consultant radiologist with experience in fetal weight estimation. The biparietal diameter (BPD) was measured from the outer table of the proximal fetal skull to the inner table of the opposite side of the skull at the level of the falx cerebri, the thalamus, and the cavum septum pellucidum.[11] Femoral length (FL) was measured using the length of the ossified diaphysis of the fetal femur from the greater trochanter to the femoral condyles.[12] Abdominal circumference (AC) was measured at the level of the junction of the umbilical vein and portal sinus around the fetal abdomen.[13] The head circumference (HC) was measured using elliptical calipers over the four points of BPD and occipital frontal diameter in the same plane as BPD, between the leading edge of the frontal bone and the outer edge of the occiput.[14] Ultrasound estimates were recorded in the study pro forma. Following the ultrasonography, the estimated weight for each fetus was calculated using the two methods as detailed below:

Shepherd method[15]

EFW = 10^(−1.7492 + 0.166 × BPD+0.046 × AC – 0.002546 × AC × BPD) (kg, cm)

Hadlock method[16]

EFW = 10^(1.3596 + 0.0064 × HC + 0.0424 × AC + 0.174 × FL + 0.00061 × BPD × AC – 0.00386 × AC × FL) (g, cm)

(FL = femur length)

Postnatal weight of babies

After delivery, the babies were weighed by the attending midwife (the midwives had adequate experience in weighing newborns) within 30 min of delivery using the same weighing scale for all babies. The interval between ultrasound estimation of fetal weight and delivery of the babies was within 48 h to minimize fetal weight changes before delivery.

Data analysis

Data were analyzed using Statistical Package for Social Sciences (SPSS®) version 20 (IBM, New York, USA). The following were calculated for each ultrasound method of estimation of fetal weight:

Mean of the error (EFW − ABW/ABW)

Mean of absolute error (absolute value of EFW − ABW/ABW)

Mean of absolute percentage error(absolute value of [EFW − ABW] ×100/ABW).

ABW = Actual birth weight

Each of the above (1, 2, 3) was calculated for actual fetal weight and categories of weight <2500 g, 2500 – <4000 g, and >4000 g.

The mean ABW and standard deviation for ABW using the two ultrasound methods was calculated. The overall correlation coefficients of the two ultrasound methods of estimation were compared.

The mean of EFW for each method and the mean of ABW were compared using the paired t-test. P < 0.05 was considered statistically significant.

Ethical approval

Ethical approval for the study was obtained from the Health Research and Ethics Committee of the University of Abuja Teaching Hospital, Gwagwalada, Nigeria. A written informed consent was obtained from all the study participants.

RESULTS

Demographics

The women were aged 21–42 years (mean 31.3 ± 7 years), parity was 0–8 (mean 4.0 ± 3.2), and maternal weight was 54.0–89.8 kg (mean 73.7 ± 14.0). The gestational age at delivery was 37–43 weeks (mean 38.6 ± 1.5 weeks) and the interval between fetal weight estimation and delivery of the baby was 2–38 h (mean 14.6 ± 8.8 h).

Weight measurements

The EFW was 1.9 kg–5.0 kg (mean 3.6 ± 0.5 kg) using the Shepard method and 1.8 kg–4.4 kg (mean 3.3 ± 0.4 kg) using the Hadlock method. The ABW was 2.0 kg–4.5 kg (mean 3.4 ± 0.5 kg).

Validation of estimated fetal weight

The mean EFW using the Shepard method was significantly higher than that of the ABW (mean 3.6 ± 0.5 kg vs. mean 3.4 ± 0.5 kg; P ≤ 0.001). Although the EFW using the Hadlock method was lower than that of the ABW, the difference did not reach statistical significance (mean 3.3 ± 0.4 kg vs. mean 3.4 ± 0.5 kg; P = 0.062) [Table 1]. The mean percentage error (mean EFW − ABW/ABW × 100) of the two methods was 5.9% for Shepard method and 2.9% for Hadlock method, indicating that the EFW by Hadlock method was closest to that by the ABW. However, the mean absolute percentage errors were similar at 12.7% and 11.5%, respectively, for the Shepard and Hadlock methods [Table 2], indicating that both methods had good accuracy in predicting birth weight.

Table 1

Comparison of estimated fetal weight and actual birth weight using the Shepard and Hadlock methods

Comparison	Paired differences			t	df	P
	Mean (kg)±SD	SE of mean	95% CI of the difference (lower – upper)
EFW by Shepard method versus ABW	0.31588±0.33311	0.02555	0.36632–0.26545	12.364	169	<0.001
EFW by Hadlock method versus ABW	0.05118±0.26282	0.02016	0.09097–0.01138	−2.539	169	0.062

CI=Confidence interval, SD=Standard deviation, SE=Standard error, EFW=Estimated fetal weight, ABW=Actual birth weight

Table 2

Accuracy of Shepard and Hadlock methods in predicting birth weight

Method	Mean of error	Percentage error	Mean absolute error (%)
Shepard	0.098	9.8	12.7
Hadlock	0.079	7.9	11.5

Macrosomia and microsomia

The Shepard method significantly overestimated macrosomia compared to that by the ABW. There was no significant difference in microsomia rate between the two methods and ABW [Table 3].

Table 3

Estimation of macrosomia and microsomia by Shepard and Hadlock methods

Method	Macrosomia			Microsomia
	Ultrasound, n (%)	At birth, n (%)	P	Ultrasound, n (%)	At birth, n (%)	P
Shepard	32 (18.8)	13 (7.6)	0.002	5 (2.9)	4 (2.4)	0.735
Hadlock	13 (7.6)	8 (4.7)	0.260	4 (2.4)	3 (1.8)	0.949

DISCUSSION

Fetal weight is the principal variable affecting fetal and neonatal morbidity and mortality. Both fetal macrosomia and intrauterine growth restriction increase the risk of perinatal morbidity, and long-term neurologic and developmental disorders.[4] Identification of intrauterine growth-restricted fetus after 37-week gestation is an indication for delivery, to reduce the risk of intrauterine fetal death. Similarly, diagnosis of fetal macrosomia especially in pelvis that has not been tried may be an indication for cesarean section.[417] Hence, accurate estimation of fetal weight has important implications for fetal well-being and management of labor and delivery.

The present study was done to evaluate the accuracy of Shepard and Hadlock methods of ultrasound prediction of fetal weight at term. In the present study, the mean ABW (3.4 kg) compared well with the weight estimated by the Hadlock method at a mean fetal weight of 3.3 kg, but the Shepard method significantly overestimated the weight at a mean fetal weight of 3.6 kg. Both methods showed good accuracy with a mean absolute percentage error of 12.7% for Shepard and 11.5% for Hadlock method. These findings are similar to other reports.[1819202122]

Considering the formulae used in the estimation of fetal weight, the Hadlock method with four biometric parameters provided more accurate results compared to the Shepard method which uses two biometric parameters. Other reports have indicated that the method with three or four biometric parameters give better EFW than methods that rely on one or two biometric parameters.[10162324] However, it has been reported that methods that are exclusively based on the measurement of AC[2526] are as accurate as methods incorporating three or four fetal biometric parameters.[2127] These differences may be related to small sample sizes in the later reports. Further, one report[23] has shown that methods that include HC in the formula produce better estimate of fetal weight. Variations in the shape of fetal head could result in erroneous estimation of fetal weight, but this is avoided by the inclusion of HC. The Hadlock method includes HC in the formula for EFW.

The present report has shown that both methods overestimated macrosomia to some extent, but significantly so by the Shepard method. Both methods were more accurate in the estimation of low birth weight or microsomia. In contrast, one report[10] has shown that these two methods underestimated fetal weight for low birth weight. This report[10] showed that the accuracy of ultrasound decreases at the extremes of birth weights, leading to overestimation in low birth weight categories and underestimation when the birth weight exceeds 4000 g. However, our finding is similar to that of Eze et al., which found that ultrasound is more accurate at predicting microsomia than macrosomia.[28]

Maternal and obstetric conditions such as amniotic fluid index, placental location, and maternal body habitus can affect some of the ultrasound methods of estimating fetal weight.[20] Shepard's method is affected by amniotic fluid index (polyhydramnios and oligohydramnios) and placental location (anteriorly located placenta).[29] One study has tried to evaluate if maternal weight has a modifying factor in the ultrasound evaluation of fetal weight, but results were inconclusive.[30] AC measurement has been found to be less accurate in the cases of oligohydramnios because the fetal skin edge may be difficult to identify when liquor volume is diminished.[31] These confounding factors were excluded in the present report.

CONCLUSSION

The present study suggests that the Hadlock method should be the preferred method for ultrasound estimation of fetal weight and is recommended for our setting and similar settings.

The small sample size in the present report and involvement of more than one midwife in weighing the infants at birth may pose limitations to the study. These should be addressed in future multicentric studies with larger sample size.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.