During pregnancy, there is a physiological increase in clotting factors such as factors I (fibrinogen), VII, VIII, IX, and X. This hypercoagulable state helps to reduce the risk of hemorrhage during delivery. Simultaneously, there is a decrease in natural anticoagulants and fibrinolysis. These changes collectively prepare the body to limit bleeding at childbirth but also increase the risk of thromboembolism. Therefore, the key coagulation change in pregnancy is the increase in clotting factors.

Reference: Williams Obstetrics, Cunningham FG, 25th Edition.