Ultrasonic fetal weight prediction: role of head circumference and femur length.

The accurate sonographic estimate of fetal weight is helpful in those instances when the fetal weight estimate might alter clinical management. Most sonographic weight predicting formulas have been based predominantly on measurements from the term fetus and then applied to the preterm fetus. Yet, the morphology of the preterm and term fetus differs considerably. The authors have examined the predictive accuracy of three published sonographic formulas in 69 preterm fetuses scanned within 48 hours of delivery. The mean birth weight was 1396 g. Thirty-nine of the infants were less than 1500 g. Sixty-two percent were products of pregnancies complicated by premature rupture of membranes. The results were compared with new equations derived from combinations of head and abdominal circumferences, biparietal diameter, and femur length obtained from the first 33 fetuses and then tested on the remaining 36. Whereas each formula correlated highly with birth weight, the selected new formula was more accurate than the published formulas by each criteria examined. In contrast to the latter, the mean error (actual minus predicted weight) of most new equations did not significantly differ from zero when tested prospectively. In addition, it appeared that the accuracy of two new formulas not incorporating femur length could be further enhanced in the group of fetuses whose femur length differed from the mean by at least 2 standard deviations by multiplying the predicted weight by the ratio of actual to mean femur length. The authors conclude that the use of head circumference and femur length coupled with a population restricted to the preterm fetus enhances the accuracy of sonographic weight predictions.