Summary: Two hundred and eighteen paired estimates of serum heat stable alkaline phosphatase (HSAP) and 24‐hour urinary oestrogen excretion were obtained from 94 inpatients during the last 10 weeks of pregnancy. There was no correlation between serum HSAP and urinary oestrogens, or between serum HSAP within 1 week of delivery and birthweight or placental weight. There was significant correlation between serum HSAP within 1 week of delivery and the placental coefficient. There was good correlation between the oestrogen excretion within 1 week of delivery and birthweight. It is concluded that serum HSAP estimations are of no value in routine obstetric care. There may be a place for serum HSAP estimations in pre‐eclamptic toxaemia.