Accurate ultrasonic estimation of fetal weight. Effect of head shape, growth patterns, and amniotic fluid volume.

Four hundred sixty-four fetuses underwent real time ultrasound examination within 72 hours before birth to evaluate other factors than the weight formula itself that might influence the accuracy of ultrasound weight estimation. The results indicated that fetal sex had no effect on weight estimation accuracy, but that fetal growth patterns, cephalic index, and the amount of amniotic fluid did. The weights of small-for-gestational age infants were significantly overestimated. Infants with oligohydramnios and low cephalic indexes were significantly underestimated by the ultrasonic weight formula. The weights of infants with high cephalic indexes were slightly overestimated and infants that were large-for-gestational age were slightly underestimated.