Comparison of fetal weight estimation formulas with and without head measurements

Most published sonographic techniques for estimating fetal weight use measurements of the biparietal diameter (BPD) and abdominal circumference. In many clinical situations, however, accurate head measurements cannot be obtained. One hundred one patients were scanned within 72 hours of delivery. Using multiple stepwise regression analysis, a best-fit formula was developed using abdominal circumference and femur length, or femur length alone. The mean error was 109 g/kg fetal weight for abdominal circumference and femur length that was comparable with BPD and abdominal circumference formulas for estimated fetal weight. The mean error when femur length was used alone was 129 g/kg fetal weight. When these models were tested prospectively on 67 patients and compared with published formulas, they yielded results that were similar in accuracy. Among these additional patients, the model using one parameter (femur length) had an average error of 114 g/kg, whereas the femur length and abdominal circumference showed a mean error of 125 g/kg.