The purpose of this study was to compare ultrasonic measurements of fetal abdominal circumference to ultrasonic measurements of fetal biparietal diameter, as a means of estimating fetal body weight. Of 58 fetuses who had abdominal circumferences measured, 48 (82%) of the predicted weights were within 15% of the actual birth weights. Forty-four of the same 58 fetuses had satisfactory biparietal diameter measurements, but only 21 (48%) of the predicted weights were within 15% of the actual birthweights. Ultrasonic measurement of abdominal circumference appears to be a more reliable index of fetal body weight than other currently available techniques.