ABSTRACT Objectives To evaluate the accuracy of gestation-adjusted birth-weight estimation using a three-dimensional (3D) fractional thigh volume (TVol) method in pregnant women with gestational diabetes mellitus (GDM), and to compare it with the conventional two-dimensional method of Hadlock et al. Methods Pregnant women with GDM were referred at 34 to 36+6 weeks’ gestation for ultrasound examination. Estimated fetal weight (EFW) was obtained using both the Hadlock and the TVol methods. Using a gestationadjusted projection method, predicted birth weight was compared to actual birth weight at delivery. Results Based on 125 pregnancies, the TVol method with gestation-adjusted projection had a mean (±SD) percentage error in estimating birth weight of −0.01±5.0 (95% CI, −0.96 to 0.98)% while the method of Hadlock with gestation-adjusted projection had an error of 1.28±9.1 (95% CI, −0.33 to 2.87)%. The mean percentage error of the two methods was significantly different (P=0.039), while the random error was not (P=1.0). For the prediction of macrosomia (birth weight ≥4000 g, n=19), sensitivity was 84 and 63% for the TVol and Hadlock methods, respectively (95% CI for difference −2 to 44%, P=0.22) and specificity was 96 and 89% for the TVol andHadlock methods, respectively (95% CI for difference 5–9%, P=0.01). Conclusions In women with GDM, a new method of estimating birth weight based on 3D-TVol measurements performed at 34+0 to 36+6 weeks’ gestation and gestation-adjusted projection of estimated fetal weight, is more accurate than the standard method based on Hadlock’s formula in predicting birth weight. The Correspondence to: Dr F. Prefumo, Maternal-Fetal Medicine Unit, Department of Obstetrics and Gynaecology, University of Brescia Piazzale Spedali Civili,1, 25123 Brescia, Italy (e-mail: email@example.com) Accepted: 1 March 2013 TVol method has comparable sensitivity but higher specificity than the Hadlockmethod in predicting neonatal macrosomia.