Birth weight prediction by Lee formula based on fractional thigh volume in term pregnancies - is it helpful?

Ultrasonographic estimation of fetal weight (EFW) is a standard obstetrical procedure in daily clinical practice. Formulas for calculating EFW most commonly are a combination of two-dimensional measurements. A relatively new approach is the use of three-dimensional measurements such as fractional thigh volume (TVol) incorporated into specific regression equations. The objective of this study was to compare the Lee formula based on three-dimensional ultrasonographic TVol in the estimation of fetal weight before delivery in term pregnancies to the Hadlock I formula.

2D/3D abdominal ultrasonography was performed in 104 women, 37-41 gestational weeks, and measurements of biparietal diameter, head circumference, abdomen circumference, and femur length, TVol were taken. Using these measurements, we compared the Lee to the Hadlock formulas in EFW. The timing of procedures was measured in 20 randomly chosen patients by an independent observer.

Mean percentage errors of formulas, Lee vs. Hadlock, were 2.13 ±9.31% vs. -2.02 ±8.79%, (p = 0.001). There was no statistically significant difference in median absolute percentage errors between the two formulas (6.09% vs. 6.10%, p = 0.56). The proportion of newborns with estimated birth weights (BW) within ±10% of actual BW was not significantly different between the two formulas (73% vs. 71%, p = 0.11). There was a significant difference in the proportion of the newborns with estimated BW within ±5% (33% vs. 42%, p = 0.000006). Statistical measurements for test performance in detecting fetuses with BW ≥ 4000 g were sensitivity 85% vs. 60%, specificity 88% vs. 96%, and accuracy 88% vs. 89%. There was no significant difference in the time to perform the measurements (69 s for Lee formula vs. 58 s for Hadlock formula, p = 0.16).

Thigh volume measurement incorporated into the Lee single parameter formula is comparable to the Hadlock I formula in terms of accuracy in predicting fetal weight before delivery. There was no significant difference in the time needed for taking necessary measurements between the two groups.