Influence of maternal body mass index on sonographic fetal weight estimation prior to scheduled delivery

Aim To investigate the possible effect of maternal obesity on the accuracy of sonographically estimated fetal weight in the third‐trimester shortly before induction of labor and to compare the accuracy of the estimation between normal weight, overweight, and class I, class II and class III obese groups. Methods This was a prospective study of singleton pregnancies with sonographic fetal weight estimation prior to scheduled delivery. Women were classified according to current body mass index (BMI) into five categories: normal (BMI 18.5–24.9 kg/m2, n = 41), overweight (BMI 25.0–29.9 kg/m2, n = 44), obese class I (BMI 30.0–34.9 kg/m2, n = 40), obese class II (BMI, 35.0–39.9 kg/m2, n = 38) and obese class III (BMI ≥ 40.0 kg/m2, n = 35). The estimated fetal weight was compared with the actual birthweight, and the difference between them was recorded as the error. Results Mean absolute error of sonographic fetal weight estimation was 106.97 ± 80.83 g, 198.88 ± 124.32 g, 248.82 ± 122.75 g, 308.31 ± 138.97 g and 446.00 ± 151.46 g in the normal, overweight, obese class I, obese class II, and obese class III groups, respectively (P < 0.001). The corresponding mean absolute percentage errors were 3.51 ± 2.76, 6.37 ± 3.91, 7.93 ± 4.81, 9.87 ± 4.32 and 14.06 ± 5.83 (P < 0.001). Conclusions Maternal obesity decreases the accuracy of sonographic fetal weight estimation. Clinicians should be aware of the limitations of sonographic fetal weight estimation, especially in obese patients.