Hemostasis undergoes major changes in normal pregnancy. These changes contribute in maintaining placental function and preventing excessive bleeding. Most changes in coagulation create a state of hypercoagulability that serves to protect the mother from hemorrhage during delivery. Unfortunately, these changes can also can predispose women to thromboembolism. It is important to note that complications with pregnancy such as pre-eclampsia, hemorrhage, infection, etc. can have multiple deleterious coagulopathies which are not specifically discussed here.
Platelets: Thrombocytopenia is the most common hemostatic abnormality observed in healthy women. Part of this is due to hemodilutional affects but increases in mean platelet volume suggests that there exists compensated platelet destruction as well.
Coagulation System: In normal pregnancy you see significant increases in factor VII, VIII, IX, X, XII, vWF and fibrinogen but relatively no changes to factors II and V. Protein C and anti-thrombin appear to be unaffected, while Protein S appears to decrease but it is unclear if this drop plays into the hypercoagulability of pregnancy
Fibrinolytic activity is reduced during pregnancy and remains low during labor and delivery. Fibrinolysis is reduced in pregnancy due to decreases in t-PA activity, which remains low until 1 hour postpartum and then returns to normal. Three weeks after delivery blood coagulation and fibrinolysis appears to generally return to normal levels.