Accuracy of estimated fetal weight assessment in fetuses with abdominal wall defects

Abstract Background Gastroschisis and omphalocele are congenital abdominal wall defects in which bowel and other abdominal contents extrude from the fetal abdominal cavity. Standard formulas for estimated fetal weight (EFW) using ultrasound include fetal abdominal circumference measurement, and have a range of error up to 10%. It is unknown whether the accuracy of EFW assessment is compromised in fetuses with abdominal wall defects due to extrusion of abdominal contents. Objectives This study aims to assess the accuracy of standard EFW assessment in fetuses with abdominal wall defects by comparing prenatal assessment of fetal weight with actual birth weight. Study Design A retrospective cohort study of fetuses diagnosed with gastroschisis or omphalocele was performed at a single center from 2012 to 2018. Fetuses with additional anomalies or confirmed chromosome abnormalities were excluded. EFW was calculated using the Hadlock formula. Published estimates of fetal growth rate were used to establish a projected EFW at birth from the final growth ultrasound, and the percent difference between projected EFW at birth and actual birthweight was calculated. The Wilcoxan rank-sum test was used to examine the difference between projected EFW and birthweight. Results We had complete data for 112 fetuses with abdominal wall defects, including 85 with gastroschisis and 27 with omphalocele. The median [IQR] projected EFW was similar to median birthweight, at 2283g [IQR 2000-2810] and 2306g [IQR 1991-2649], respectively, which did not represent a statistically significant difference between projected EFW and birthweight (p=.32). The median percent error was 6.8 [3.1-12.8]. We also did not find statistical differences when comparing projected EFW and birthweight in patients with gastroschisis (p=.52) or omphalocele (p=.35) individually. EFW was underestimated in a majority of cases (n=68, 60.7%). Conclusions In fetuses with abdominal wall defects, standard measurement of fetal weight shows accuracy that is at least comparable to previously established margins of error for ultrasound assessment of fetal weight. Standard EFW assessment remains an appropriate method of estimating fetal weight in these fetuses.