Fetal head and abdomen circumference measurements have been made using pulsed ultrasound techniques. The mean head to abdomen (H/A) circumference ratio with 95% confidence limits was determined in 568 normal pregnancies from 17 to 41 weeks menstrual age. The mean ratio was 1-18 at 17 weeks, but decreased slowly until 29 weeks when the ratio was 1-11; thereafter there was a sharp fall in the mean ratio to 1-01 at 36 weeks and 0-96 at 40 weeks. The H/A circumference ratio was also determined in 31 small-for-dates fetuses within one week of delivery. In all cases the fetal weight as predicted from the fetal abdomen circumference measurement was below the 5th centile weight for gestation. The ratio was above the 95th centile limit in 22 (71%) of these fetuses and evidence is presented to suggest that the H/A circumference ratio can be used to distinguish between symmetrical and asymmetrical growth retardation.