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 post-partum period.