SUMMARY: 1. The association between foetal growth and placental weight according to sex and order of birth is examined in single pregnancy in man. Data recorded are: birth weight (on 13,020 births); birth weight and placental weight (on 4931); and weights at birth and at 3 and 6 months after birth (on 405). 2. From 30 to 31 weeks' gestation, the earliest period examined, males are heavier than females. There is little difference between mean weights of first and later born until 36–37 weeks, from which time later born are heavier than first born (Fig. 1). 3. From 34 to 35 weeks (the earliest period from which numbers permit the comparison) mean placental weights are greater for males than for females, and for later born than for first born (Fig. 2). 4. At a given weight of placenta males are heavier than females (Fig. 3); there is little difference between weights of first and later born at placental weights under 1½ lb., but at higher placental weights later born are heavier than first born (Fig. 4). 5. The difference between growth rates of males and females is of course determined genetically. It is suggested that it may be explained by the observations that (a) the placenta is larger for males than for females, and (b) males make a greater use than females of a unit weight of placenta. The sex difference in growth rate is exhibited after birth (Fig. 5). 6. The difference between foetal growth rates of first and later born is determined by the pre-natal environment. It may be explained largely, though not entirely, by the fact that the placenta is larger for later born than for first born. The difference in growth rate is reversed after birth, and at 6 months first born are as heavy as later born (Fig. 5).