Effect of Increased Body Mass Index on the Accuracy of Estimated Fetal Weight by Sonography in Twins

Objective. The purpose of this study was to determine whether increasing body mass index (BMI) decreases the accuracy of sonographic estimations of fetal weight in twin gestations. Methods. A chart review was conducted, in which 361 charts of patients with twin gestations over a 2-year period were reviewed. A total of 194 patients had sonographic examinations for fetal weight within 6 days of delivery and were included in the analysis. The difference between the sonographically estimated fetal weight was compared with the actual birth weight for each twin and stratified for the patient's BMI. Results. There was a significant increasing trend in mean absolute percent errors with increasing BMIs in both twins (P < .05). The mean absolute percent errors for twin A were 6% for patients with a BMI of less than 25 and 9% for those with a BMI of greater than 30. The mean absolute percent errors for twin B were 6.7% for patients with a BMI of less than 25 and almost 11.7% for those with a BMI of greater than 30. There was a significantly increasing trend in mean absolute differences in grams for both twins with increasing gestational age, with almost a 4-fold increase from less than 28 weeks to greater than 36 weeks in both twins (P < .05). Conclusions. Increasing maternal obesity decreases the accuracy of sonographically determined fetal weight in twin gestations, particularly for twin B.