Accuracy of estimated fetal weight in extremely preterm infants and the impact of prepregnancy body mass index.

BACKGROUND

Antenatally, we rely on ultrasound estimated fetal weight as a proxy for birthweight to inform discussions regarding perinatal morbidity and mortality. Maternal obesity may negatively impact the quality of ultrasound imaging, and thus, understanding the associations between obesity and estimated fetal weight in the preterm period is important.

OBJECTIVE

Given the rising obesity rates and association with preterm birth, we sought to determine the accuracy of ultrasound-derived estimated fetal weight in predicting birthweight in preterm infants by prepregnancy body mass index and to evaluate the accuracy of estimated fetal weight in predicting birthweight between small-for-gestational-age and appropriate-for-gestational-age infants.

STUDY DESIGN

We included all women who delivered a live-born singleton infant between 23 0/7 and 31 6/7 weeks of gestation and had an ultrasound estimated fetal weight within 7 days before delivery. We calculated the mean percentage difference between estimated fetal weight and birthweight and the absolute percent difference. Excess error was defined as an absolute percentage difference of >20%. We used multivariable modified Poisson models to determine the association between prepregnancy body mass index and small for gestational age and excess ultrasound error.

RESULTS

Our cohort included 641 infants with a mean gestational age of 28.0±2.6 weeks and a mean birthweight of 1110±425 g. More than one-third of our cohort were obese (227 [35%]). The mean percentage difference between estimated fetal weight and birthweight was 7.7%±11.2% among all infants. Ultrasound overestimated birthweight in 77% of the cohort (n=492). Stratified by body mass index, the mean percentage differences between estimated fetal weight and birthweight were 6.7%±11.0% in women with normal weight and 9.5%±12.0% in women with obesity (P=.02). The mean percentage differences between estimated fetal weight and birthweight were 11.0%±11.0% in small-for-gestational-age infants (n=80) and 7.1%±11.0% in appropriate-for-gestational-age infants (P<.001). Small-for-gestational-age infant was associated with an increased risk of excess ultrasound error with an adjusted relative risk of 2.3 (95% confidence interval, 1.2-4.3).

CONCLUSION

Although ultrasound estimated fetal weight overestimated birthweight, particularly in small-for-gestational-age infants, most estimates were within 10% of actual birthweight. Obesity and small-for-gestational-age birth were both associated with an increased risk of excess ultrasound error (≥20%) in estimating birthweight.