The objective of the study was to determine the changes in the plasma activities of protein C and protein S that occur during normal pregnancy. In this prospective cross-sectional study, plasma activities of protein C and protein S were measured in 32 normal pregnant women in the first, second and third trimester and 6 weeks after delivery; There was a significant fall in protein C and protein S activities during normal pregnancy compared with the post-puerperal period. The activities of protein C and protein S also gradually decreased throughout pregnancy (p < 0.01). Increasing plasma volume during normal pregnancy and its dilutional effect might play some role in the low activities of protein S observed. The normal falls in protein S and protein C activities make it difficult to diagnose protein S and C deficiency during pregnancy. Based on our findings, if a woman has a thromboembolic event during pregnancy; testing for a definitive diagnosis of protein C or protein S deficiency or functional failure should be delayed until at least 6 weeks postpartum.