Does Maternal Body Mass Index Have an Effect on the Accuracy of Ultrasound-Derived Estimated Birth Weight?: A Retrospective Study.

Objectives The purpose of this study was to investigate the relationship between the maternal body mass index (BMI) and the accuracy of ultrasound-derived birth weight. Methods A retrospective chart review was performed on women who had an ultrasound examination between 36 and 43 weeks' gestation and had complete delivery data available through electronic medical records. The ultrasound-derived fetal weight was adjusted by 30 g per day of gestation that elapsed between the ultrasound examination and delivery to arrive at the predicted birth weight. Results A total of 403 pregnant women met inclusion criteria. Age ranged from 13–44 years (mean ± SD, 28.38 ± 5.97 years). The mean BMI was 32.62 ± 8.59 kg/m2. Most of the women did not have diabetes (n = 300 [74.0%]). The sample was primarily white (n = 165 [40.9%]) and Hispanic (n = 147 [36.5%]). The predicted weight of neonates at delivery (3677.07 ± 540.51 g) was higher than the actual birth weight (3335.92 ± 585.46 g). Based on regression analyses, as the BMI increased, so did the predicted weight (P < .01) and weight at delivery (P < .01). The accuracy of the estimated ultrasound-derived birth weight was not predicted by the maternal BMI (P = .22). Maternal race and diabetes status were not associated with the accuracy of ultrasound in predicting birth weight. Conclusions Both predicted and actual birth weight increased as the BMI increased. However, the BMI did not affect the accuracy of the estimated ultrasound-derived birth weight. Maternal race and diabetes status did not influence the accuracy of the ultrasound-derived predicted birth weight.