Biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL) were compared by race and sex in 5,405 ultrasound examinations done on 2,831 women. Black fetuses had significantly longer FL than white fetuses; male fetuses had larger BPD, HC and AC than females. The differences in BPD, HC and AC correlated with the different birth weights observed between male and female infants, 3,253 vs. 3,153 g. The difference in birth weight between black and white infants, 3,152 vs. 3,331 g, did not correlate with differences in their respective BPD, HC and AC. Earlier delivery accounted for some but not all of the birth weight difference between black and white infants. Our data suggest that there may be differences in body length proportions (longer legs and shorter trunks in black infants) that are important factors in understanding the birth weight difference between black and white infants. Furthermore, fetal race and sex differences could account for some degree of error in the ultrasound estimation of gestational age.