How does the coagulation system change in pregnancy, and why?

Pregnancy is a hypercoagulable state: clotting factors (especially fibrinogen and factors VII, VIII, X) increase, fibrinolysis decreases, and there is venous stasis (Virchow's triad).

This is a physiological preparation to reduce blood loss at delivery, but it markedly increases the risk of venous thromboembolism in pregnancy and the puerperium.

Clinical relevance. The hypercoagulable state protects against delivery haemorrhage but raises VTE risk, justifying thromboprophylaxis.

More Detail

Coagulation changes in pregnancy

Pregnancy shifts the haemostatic balance toward clotting - protective for delivery but raising thrombotic risk.

The changes (hypercoagulable state)

• Increased clotting factors - notably fibrinogen (and factors VII, VIII, X, von Willebrand factor).
• Reduced fibrinolysis (raised PAI-1/PAI-2).
• Reduced protein S activity (acquired resistance to activated protein C).
• Venous stasis (vasodilation, uterine compression of pelvic veins).

Why

• A physiological preparation to limit blood loss at placental separation/delivery.

Consequence

• Completes Virchow's triad (hypercoagulability, stasis, and endothelial injury at delivery), markedly increasing the risk of venous thromboembolism (DVT/PE) - a leading cause of maternal death.
• Hence the importance of VTE risk assessment and thromboprophylaxis in pregnancy and the puerperium.