Regression analysis was used to develop an in utero fetal weight model from a population of 392 predominantly middle-class white patients with certain menstrual histories. There was a gradual increase in fetal weight from 35 g at 10 weeks to 3,619 g at 40 weeks, with uniform variance of +/- 12.7% (1 standard deviation) throughout gestation. When tested against the estimated weights of 1,771 chromosomally normal fetuses between 14 and 21 weeks, the mean percent difference was 0.8% and the average absolute percent error was 3.3%. When compared with actual delivery data for 163 fetuses in the group, the mean percent difference was 0.8% and the average absolute percent error was 1.1%. These data are compared with other prenatal weight curves obtained at ultrasound and with data from several large postnatal weight studies.