Third trimester ultrasound for fetal macrosomia: optimal timing and institutional specific accuracy.

Purpose: To determine the performance of third trimester ultrasound in women with suspected fetal macrosomia.
Materials and Methods: We performed a retrospective cohort study of fetal ultrasounds from January 2004 to December 2014 with estimated fetal weight (EFW) between 4000 and 5000 g. We determined accuracy of birth weight prediction for ultrasound performed at less than and greater than 38 weeks, accounting for diabetic status and time between ultrasound and delivery.
Results: There were 405 ultrasounds evaluated. One hundred and twelve (27.7%) were performed at less than 38 weeks, 293 (72.3%) at greater than 38 weeks, and 91 (22.5%) were performed in diabetics. Sonographic identification of EFW over 4000 g at less than 38 weeks was associated with higher correlation between EFW and birth weight than ultrasound performed after 38 weeks (71.5 versus 259.4 g, p < .024). EFW to birth weight correlation was within 1.7% of birth weight for ultrasound performed less than 38 weeks and within 6.5% of birth weight for ultrasound performed at greater than 38 weeks.
Conclusions: Identification of EFW with ultrasound performed less than 38 weeks has greater reliability of predicting fetal macrosomia at birth than measurements performed later in gestation. EFW to birth weight correlation was more accurate than previous reports.