At term birth, boys are heavier than girls. This difference is thought to be generated in part by androgen action; its time course has not been deciphered. Androgen action may not only increase weight gain,but may also alter its time course. We have tested this hypothesis by examining the difference in gestational age of 281,894 boys and girls with weights between 500-4,749 g, The age at which children are born with a given weight was found to depend on gender: boys were consistently younger than girls (p < 0.001), the age difference being most pronounced in the lower birth weight classes. Thus, the gender difference in fetal growth appears to be rather pronounced before the third trimester and relatively less marked towards term. In conclusion, the male conceptus seems to grow not only more, but also earlier than the female. Hence, some critical time windows of development may be slightly different in boys and girls, and this phenomenon may be one of the bases for gender differences in the sensitivity to fetal programming. Copyright (C) 1999 S. Karger AG, Basel.