Activated protein C (APC) is a strong inhibitor of coagulation, inactivating coagulation factors Va and VIIIa upon binding to protein S (PS) in the presence of thrombin and thrombomodulin. The normal concentration of PC in the plasma is approximately 4 μg/ml. Throughout pregnancy, PC activity and antigenic levels show no significant trend and remain within the normal reference range. Several PC point mutations have been documented, including those characterized as type I and II PC deficiencies. These, as well as mutations in Protein S (PS), factors Va and VIIIa, and thrombomodulin, can result in venous thromboses of various degrees of severity. The relative risk of thrombosis with PC deficiency is 7.3%. In pregnancy, the risk is 3-10% antepartum and 7-19% postpartum. PC deficiency has also been reported to be associated with both non-recurrent and recurrent first, second and third trimester miscarriages, intrauterine fetal death, intrauterine growth retardation, placental abruption and preeclampsia. However, prior to 10 weeks of gestation, no significant relationship between PC deficiency and pregnancy loss has been established.