Background: We analyzed the changes in protein C and protein S activities in each trimester and postpartum period in pregnant Korean women and their association with adverse pregnancy outcomes. Methods: This prospective study included 172 singleton pregnant women and 53 non-pregnant women. Protein C and protein S activities were measured using chromogenic analysis, and differences between each group and the relationship of the protein activities with adverse pregnancy outcomes were analyzed. Results: Protein C activity was significantly increased in the second and third trimesters and in the postpartum period (P < 0.001, P =0.017, and P <0.001, respectively), whereas protein S activity was significantly decreased throughout pregnancy compared to that in the non-pregnant group (P <0.001, P < 0.001, P < 0.001, and P =0.011, respectively). In the third trimester of pregnancy, protein C activity was significantly higher in women who delivered babies with fetal growth restriction (40.1% vs. 27.6%, P =0.040). Protein S activity was significantly lower in pregnant women who developed pregnancy-induced hypertension/preeclampsia and gestational diabetes than in women who did not develop these complications (10.1% vs. 35.0%, P =0.008; and 25.9% vs. 38.8%, P =0.042, respectively). Conclusions: Protein C activity tended to increase during pregnancy and the postpartum period compared to that in the non-pregnant period. Protein S activity decreased rapidly from the first trimester and continued decreasing during pregnancy and the postpartum period. Reduced protein S activity in the third trimester is associated with pregnancy-induced hypertension and preeclampsia.