Accuracy of ultrasound-derived estimated fetal weight in a cohort of periviable births

Objective As neonatal resuscitation is offered at increasing extremes of prematurity, ultrasound-derived estimated fetal weight (EFW) is a crucial data point in patient counseling. We aimed to evaluate the accuracy of EFW compared to birthweight (BW) among periviable neonates, and identify factors associated with accuracy.Methods Medical records were abstracted for all neonates born between 22 weeks 0/7 days and 24 weeks 6/7 days at a single tertiary center between January 2013 and May 2024. Liveborn, non-anomalous neonates with an ultrasound-derived EFW within 7 days of delivery were included. The primary outcome was absolute percent difference between Hadlock 3 EFW and BW (“accuracy”), with BW as the primary predictor. Multivariable linear mixed effects regression was performed including maternal BMI and variables associated with accuracy in univariate analyses at p < 0.1.Results 141 neonates from 129 pregnancies were included. Median gestational age at delivery was 23 4/7 weeks, median BW was 560 g [IQR 480, 640], and median time from EFW to delivery was 2 days [IQR 1–3 days]. Over half (55%) of pregnant individuals had BMI >30. Median accuracy was 7.1% [IQR 2.9%, 12.2%]. EFW was within 10%, 15% and 20% of BW in 65.3%, 82.3% and 92.2% of cases, respectively. In univariate analyses, decreasing BW was associated with reduced accuracy (β = −0.012 [−0.02, −0.0002], p = 0.04), and multiple gestations were associated with increased accuracy (median accuracy 3.1% among fetuses with a twin versus 7.9% among singletons; p = 0.002). Maternal BMI was not associated with accuracy (p = 0.08). In multivariable analyses accounting for multiple gestations, BW remained inversely associated with accuracy (β = −0.014 [−0.027, −0.001], p = 0.03). Oligohydramnios was also associated with reduced accuracy (β = 3.47 [1.03, 5.91], p = 0.005).Conclusions Hadlock ultrasound-derived EFW is an overall reasonable predictor of BW among periviable deliveries, but demonstrated reduced accuracy among extremely low birthweight neonates and in the setting of oligohydramnios.