Ultrasound evidence of sexual difference in fetal size in first trimester.

Boys born at term weigh on average 50-150 g more than girls and are 0 5-1 0 cm longer.' 2 At 1 year of age these differences have;kncreased to 0-5 kg and 1-2 cm.' Conversely, among babies born prematurely a weight difference between the two sexes can be detected only in the last eight to 12 weeks before term.' These observations have led to the assumption that the sexual difference in weight and size is hormone dependent. In order to establish local normal tables of fetal growth a series of normal pregnancies were studied by ultrasonic scanning. Quite unexpectedly, male fetuses were already on average larger than female ones at the first measurement in the 8th to 12th week. Therefore a genetic rather than a hormonal mechanism is probably behind the sexual difference in fetal size.