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/m(2), n = 41), overweight (BMI 25.0-29.9 kg/m(2), n = 44), obese class I (BMI 30.0-34.9 kg/m(2), n = 40), obese class II (BMI, 35.0-39.9 kg/m(2), n = 38) and obese class III (BMI ≥ 40.0 kg/m(2), 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.