Accuracy of the Final Growth Ultrasound in Predicting Birthweight Centile in Women With Class III Obesity: A Retrospective Cohort Study

Introduction This study evaluated agreement between final third-trimester ultrasound estimated fetal weight (EFW) centile and subsequent birthweight centile in singleton pregnancies among women with class III obesity (body mass index (BMI) ≥40 kg/m²). Secondary objectives were to assess antenatal detection of small-for-gestational-age (SGA) and large-for-gestational-age (LGA) infants and associations of maternal BMI and scan-to-delivery interval with centile prediction error. Methods We conducted a retrospective cohort study of singleton pregnancies in women with booking BMI ≥40 kg/m². Agreement between final ultrasound EFW centile and birthweight centile was assessed using centile differences, correlation, SGA/appropriate-for-gestational-age (AGA)/LGA categorical agreement, and weighted Cohen's kappa. Secondary analyses assessed diagnostic performance for SGA and LGA and associations of maternal BMI and scan-to-delivery interval with centile error. Results A total of 245 pregnancies had complete data. Mean maternal BMI was 44.8 kg/m², and mean scan-to-delivery interval was 14.4 days. EFW centile was positively correlated with birthweight centile (r = 0.732, p < 0.001). The median absolute centile difference was 13.1 centiles, and the mean signed difference was +3.6 centiles (95% confidence interval (CI) 1.0-6.2). Overall SGA/AGA/LGA category agreement was 82.4% (weighted κ = 0.502). Sensitivity was 26.9% for SGA and 46.9% for LGA, with specificities of 99.5% and 97.2%, respectively. Absolute centile error did not differ significantly across BMI or scan-to-delivery interval groups. Conclusions Final third-trimester ultrasound EFW centile was positively correlated with birthweight centile in women with class III obesity, but clinically important individual discrepancies remained. The final scan had high specificity but low sensitivity for identifying infants subsequently born SGA or LGA. Increasing BMI within the class III obesity range and scan-to-delivery interval were not significantly associated with greater centile error.