Influence of maternal body mass index on the accuracy of clinical and sonographic fetal weight estimation

Objective: Estimated fetal weight (EFW) is essential for managing pregnancy and delivery. Currently, two primary methods are used for EFW: Clinical and sonographic estimation. Both might be influenced by maternal habitus. Obesity has become a global epidemic; however, its effect on EFW and the accuracy of the two methods has yet to be determined. The aim of the present study was to describe the accuracy of clinical EFW (cEFW) and sonographic EFW (sEFW) as a function of maternal body mass index (BMI, calculated as weight in kilograms divided by the square of height in meters) by comparing them to actual birth weight. Methods: This was a retrospective cohort study at a single tertiary center (2014–2020), including term (37–42 weeks) singleton deliveries with documented BMI, cEFW, and sEFW performed within 10 days of delivery, and known birth weight. At our institution, cEFW is routinely performed at admission by trained obstetricians as part of a standardized Leopold-based assessment. Participants were stratified into BMI groups: <25, 25–29.9, 30–34.9, and ≥35. The primary outcome was estimation accuracy across BMI strata, expressed as absolute error (in grams [g]) and as deviations greater than 10% and 20%. Secondary outcomes included diagnostic performance for large-for-gestational-age (LGA) neonates, assessed using receiver operating characteristic (ROC) curves and the area under the curve (AUC). Multivariable logistic regression identified factors associated with an error rate exceeding 20%. Results: Among 19 397 deliveries, 2182 women qualified for analysis. Participants were distributed across BMI groups as follows: <25 (n = 408), 25–29.9 (n = 809), 30–34.9 (n = 634), and ≥35 (n = 331). Mean birth weight rose progressively with BMI, from 3045 ± 519 g in the lowest BMI group to 3557 ± 453 g in the ≥35 group (P < 0.01). Across BMI ≥25, cEFW showed larger absolute error than sEFW: BMI 25–29.9 (222 ± 184 vs. 202 ± 156 g), 30–34.9 (238 ± 187 vs. 204 ± 154 g), and ≥35 (254 ± 207 vs. 211 ± 161 g) (all P < 0.01). Rates of sEFW deviation >20% were low and stable across BMI (0.6%–1.7%; P = 0.42), whereas cEFW >20% increased with BMI (2.0%–6.0%; P < 0.01). In adjusted models, cEFW had higher odds of >20% error that escalated with BMI (adjusted odds ratio [aOR] 3.34, 5.87, 9.17 for BMI 25–29.9, 30–34.9, ≥35; all P < 0.01). In adjusted models, maternal BMI was not associated with >20% error in sEFW. For LGA detection, sEFW outperformed cEFW across strata, with the most significant gap at BMI ≥35 (AUC 0.873 vs. 0.791). Conclusion: Maternal obesity significantly reduces the accuracy of clinical EFW, whereas sonographic EFW maintains stable performance across BMI strata. Sonographic assessment should therefore be prioritized for fetal weight estimation in women with elevated BMI, particularly in the late third trimester, when weight-based delivery decisions are made.