ObjectiveTo compare the accuracy of three-dimensional ultrasound-assessed fetal thigh volumetry in predicting birth weight with that of other commonly used formulas composed of biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL) by two-dimensional ultrasound.MethodsWe assessed the thigh volume of 100 fetuses using three-dimensional ultrasound. Meanwhile, their BPD, AC, and FL were measured by two-dimensional ultrasound. All infants were delivered within 48 hours after the ultrasound examinations. From polynomial regression analysis, we generated a best-fit formula for the thigh volume to predict birth weight. The accuracy of this thigh-volume formula was compared with those of three formulas commonly used in the United States. In addition, another group of 50 fetuses was measured for prospective validation.ResultsThe thigh volume assessed by three-dimensional ultrasound was highly correlated with birth weight (r= 0.89,n= 100,P< .0001). The best-fit formula for thigh volume to predict birth weight was linear, and it was superior to the other commonly used two-dimensional formulas in predicting birth weight. The predicting error (0 g), percent error (0.7%), absolute error (176.1 g), and absolute percent error (5.8%) of the thigh-volume formula were all smaller than those of the other formulas (n= 100, allP< .05). In addition, the thigh-volume formula predicted birth weight more accurately than the other two-dimensional formulas in the prospective-validation group. The three-dimensional formula had smaller mean values of predicting error (38.6 g), percent error (1.5%), absolute error (160.0 g), and absolute percent error (5.1%) than the two-dimensional formulas (n= 50, allP≤ .001), as well as the smallest variances of the above errors (178.1 g, 5.6%, 84.3 g, and 2.9%, respectively).ConclusionThe three-dimensional ultrasound-assessed thigh volume has better accuracy in predicting birth weight than the commonly used formulas by two-dimensional ultrasound, and it may improve fetal weight prediction in clinical practice. However, a large-scale prospective validation study may be needed to confirm our conclusions.