Objective: To compare the accuracy between the new equation of combined three-dimensional ultrasonography [3D-US] fractional thigh volume [Tvol] with biparietal diameter [cTvol-BPD] and formula of two-dimensional ultrasonography [2D-US] for estimating fetal weight.

Materials and Methods: The prospective cross sectional study was conducted in 251 singleton pregnancies at 37 to 42 weeks of gestation. Sonographic evaluation was performed within 48 hours before delivery. The fetal biometry was performed using 2D-US, while fractional Tvol was performed using 3D-US. The estimated fetal weight [EFW] was calculated using both Hadlock’s formula and the new equation from the previous study of Tantechasatid; birth weight (grams) = 1,241.285 + 22.908 Tvol + 43.741 BPD. Paired t-test, Pearson’s correlation coefficient and intraclass correlation coefficient [ICC] were used for data analysis.

Results: The mean fetal weight was 3,142.3 grams (SD 387.6). Birth weight calculated from cTvol-BPD and 2D-US were correlated to actual birth weight (r = 0.9 and 0.8, respectively). The mean difference between cTvol-BPD and actual birth weight was 96.6 grams (SD 194.6, p<0.001), meanwhile 2D-US was 98.5 grams (SD 283.9, p<0.001). Birth weight calculated from new equation was more accurate than Hadlock’s formula, ICC 0.8 (95% CI 0.7 to 0.9), p<0.001) and 0.7 (95% CI 0.6 to 0.8, p<0.001), respectively.

Conclusion: The new equation using combined 3D-US fractional Tvol with BPD was more accurate than 2D-US in estimating fetal weight at 37 to 42 weeks of gestation.

Keywords: Fractional thigh volume, 2-D ultrasonographic biometry, Estimated fetal weight