Accuracy of sonographic estimated fetal weight in suspected macrosomia: the likelihood of overestimating and underestimating the true birthweight.

OBJECTIVE: Macrosomia has increased risk of serious adverse outcomes for both infants and their mothers. As such, many providers recommend induction of labor or cesarean delivery (CD) based on sonographic estimated fetal weight (sonoEFW) cutoffs. It is known that sonoEFW is a poor predictor of birthweight (BW), especially at the extremes of weight. It is not clear, however, whether sonoEFWs tend to underestimate or overestimate the true BW among fetuses with suspected macrosomia. The objective of this study was to compare rates of overestimation of BW among women with suspected macrosomia by sonoEFW.

METHODS: This was a retrospective cohort study of women who presented to a single maternal-fetal medicine ultrasound unit within 2 weeks prior to delivery from January 2011 to November 2017. We identified women who received a sonoEFW ≥4000 g. The study sample was divided into four sonoEFW categories: 4000-4249, 4250-4499, 4500-4749, and ≥4750 g. Accuracy of sonoEFW was compared across groups, with the primary outcome being overestimation of BW.

RESULTS: A total of 502 patients were included, of whom 301 (60.1%) had a sonoEFW 4000-4249 g, 135 (26.9%) had a sonoEFW 4250-4499 g, 45 (9.0%) had a sonoEFW 4500-4749 g, and 21 (4.2%) had a sonoEFW ≥4750 g. In each sonoEFW group, the risk of overestimating BW was greater than 50%, and the likelihood of overestimation of BW increased significantly across sonoEFW groups (69.4, 76.3, 80.0, 95.2%, p < .001). This held true after adjusting for differences in baseline characteristics, including diabetes and amniotic fluid index. BW ≥4500 g was not accurately predicted. Among women with sonoEFW 4500-4749 g, only 28.9% delivered a neonate with a BW >4500 g; for women with a sonoEFW ≥4750 g, only 47.6% had a BW >4500 g. One hundred sixty-one (32.1%) women underwent CD for suspected macrosomia. Of these CDs, 48 (29.8%) of neonates had a BW <4000 g and 134 (83.2%) had a BW <4500 g.

CONCLUSION: In patients undergoing sonoEFW within 2 weeks of delivery, sonoEFWs ≥4000 g are significantly more likely to overestimate than underestimate the true BW. Obstetricians should be cautious about intervening based on sonoEFW alone, given the high risk that this value is an overestimation of the true weight.