Pre-delivery BMI and the Accuracy of Fetal Weight Estimation in Very Preterm Infants

To examine the relation between maternal pre-delivery BMI and the accuracy of sonographic estimated fetal weight (EFW) in very preterm infants (<32 weeks gestation).

This retrospective study included singleton infants born between January 2010 and March 2023, at gestational ages 23⁰ to 31⁶ weeks, at a tertiary university-affiliated hospital. Absolute weight, percentage error, absolute percentage error, and overestimation and underestimation of EFW were compared between women with pre-delivery normal weight (BMI 18.5-24.99 kg/m²), overweight (BMI 25.0-29.99 kg/m²), and obesity (BMI >35.0 kg/m²). Multivariate linear regression analyses adjusted for potential confounders were performed to assess relations of maternal pre-conception and of pre-delivery BMI, with EFW accuracy.

Included were 286 pregnancies. The absolute difference, percentage error, absolute percentage error, error within the 10% range, and underestimation or overestimation of EFW were similar between the groups. The multivariate linear regression analyses did not show significant associations of pre-conceptional BMI or of pre-delivery BMI with the percentage error. However, for small for gestational age compared to appropriate for gestational age fetuses, the percentage error was greater (8.9% vs. -0.6%, β = 0.35, P < 0.001) and the absolute percentage error was greater (11.0% vs. 6.7%, P < 0.001). Small for gestational age fetuses were at risk of fetal weight overestimation (percentage error exceeding 15%); OR 7.20 (95% CI 2.91-17.80).

Maternal pre-delivery BMI was not found to be related to EFW accuracy in very preterm infants. Nevertheless, EFW should be interpreted carefully, as it may underdiagnose poor fetal growth in this population.