Summary. We measured activated protein C (APC) anticoagulant activity in 20 healthy women at 14-20, 28 and 36 weeks gestation, and at 1 d post-partum. Significant reductions in the mean APC sensitivity ratio (APC-SR) were observed at all stages of pregnancy studied compared with the mean APC-SR obtained for baseline measurements carried out at > 8 weeks post-partum. APC resistance was seen in 8/19 (42%) and in 11/20 (55%) women at 14-20 and 28 weeks gestation respectively. The development of resistance to APC may contribute to the increased risk of thrombosis during pregnancy.