(452) Accuracy of ultrasound-derived estimated fetal weight in peri-viable 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 accuracy of EFW compared to neonatal birthweight (BW) in peri-viable births and identify factors associated with accuracy.

Study Design: Medical records were abstracted for all liveborn, non-anomalous neonates born at 22w0d-24w6d at a single tertiary institution between 1/2013-5/2024, with ultrasound-derived EFW within 7 days of delivery. Primary outcome was absolute percent difference (accuracy) between EFW (Hadlock) and BW. Linear regression and Kruskall-Wallis tests were used to assess associations of accuracy with imaging variables, maternal and pregnancy characteristics. Adjusted linear regression was performed including maternal BMI at EFW (an a priori plausible association) 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 23w4d, with median BW 560g. Most deliveries (83%) were sequelae of cervical insufficiency, preterm labor, or preterm prelabor rupture of membranes; 17% were iatrogenic for maternal or fetal indications. Median time from EFW to delivery was 2 days. A certified sonographer performed 85% of ultrasounds. EFW was highly correlated with BW (p< 0.001, R²=0.72); 54% of EFWs underestimated BW. Median absolute percent difference was 7.1% [IQR 2.9%,12.2%]. In unadjusted analyses, increasing BW (p=0.04) and multiple gestations (p=0.002) were associated with improved accuracy; oligohydramnios was associated with decreased accuracy (p=0.04). Maternal BMI, sonographer level, fetal presentation and EFW timing were not associated with accuracy. Adjusting for BMI, oligohydramios and multiples, BW remained inversely associated with EFW accuracy (coefficient -0.016, p=0.013).

Conclusion: Ultrasound-derived EFW was an excellent predictor of BW among peri-viable deliveries at our institution. Accuracy declined with lower BW, which may be considered in antenatal counseling.