Normal pregnancy is accompanied by changes in the coagulation and fibrinolytic systems. These include increases in a number of clotting factors (I, II, VII, VIII, IX and XII), a decrease in protein S levels and inhibition of fibrinolysis. As gestation progresses, there is also a significant fall in the activity of activated protein C, an important anticoagulant. While these physiological changes may be important for minimizing intrapartum blood loss, they entail an increased risk of thromboembolism during pregnancy and the postpartum period.
Bremme K. Haemostatic changes in pregnancy. Best Practice & Research Clinical Haematology. 2003;16(2):153-168View Article