The changes in the coagulation system in normal pregnancy are consistent with a continuing low grade process of coagulant activity. Using electron microscopy, fibrin deposition can be demonstrated in the intervillous space of the placenta and in the walls of the spiral arteries supplying the placenta. Normal pregnancy is accompanied by dramatic changes in the coagulation and fibrinolytic systems, with a marked increase in some of the procoagulant factors, particularly fibrinogen, and suppression of fibrinolysis. Certain disease states specific to pregnancy have profound effects on platelet consumption, lifespan and function. A fall in the platelet count may precede any detectable rise in serum fibrin degradation products (FDP) in women subsequently developing pre-eclampsia. The most common platelet abnormality encountered in clinical practice is thrombocytopenia. During healthy pregnancy there is little change in antithrombin level but there is some decrease at parturition and an increase in the puerperium.