Computer-assisted evaluation of ultrasonic fetal weight prediction using multiple regression equations with and without the fetal femur length.

Ultrasonic fetal femur length (FL) measurement provides an accurate fetal length. Combined with cross-sectional dimensions such as the biparietal diameter (BPD) and abdominal circumference (AC), FL provides a three-dimensional profile of the fetus. The usefulness of incorporating FL measurements into weight-predicting regression equations containing the BPD and AC was assessed. Multiple regression equations containing these three parameters as a function of log10 birthweight were constructed from data obtained from 125 patients within 48 hours of delivery. The mean errors in fetal weight prediction derived from five of these equations were assessed in another 92 patients. The coefficients of multiple correlation were marginally better in those equations that contained FL. The mean differences were also slightly lower with these equations compared with those that did not contain FL. However, the mean differences derived from all the equations were not significantly different from each other (P greater than 0.05). Therefore, despite theoretical considerations, the results from this study do not justify the routine clinical use of these new equations containing FL to estimate intrauterine fetal weight, in replacement of existing charts based on BPD and AC.