Abstract： Objective To determine the relative factors in predicting birthweight and to evaluate the usefulness of the fetal abdominal circumference measurement by ultrasound in prediction of macrosomia. Methods One hundred and forty eight pregnant women whose sum of the fundus height plus the abdominal circumference was greater than 135 cm were chosen, the biparietal diameter (BPD), abdominal circumference (FAC), femus length (FL) of their fetus within 3 days before delivery were measured by ultrasound. At the same time, the fundus height of the uterus、 abdominal circumference、 the gestational age、 the body weight gain during pregnancy、 and the body height of the mother were obtained. Stepwise multiple regression analysis was used to determine the relation ship between birth weight and these factors. According to the birth weight two groups were divided, macrosomic and nonmacrosomic group. The characters of the two groups were compared. Results The best linear predictor of birth weight was ultrasound measurement of fetal abdominal circumference (FAC), which had a correlation coefficient of 0.85. An FAC of 36 cm or more can predict 82% of macromic infants. The cesarean section rate of the macrosomic group was 70.2%. Conclusions Fetal abdominal circumference measurement by ultrasound is the best predictor of birth weight among the parameters. FAC is useful in screening for suspected macrosomia.