Physiologic and anatomic changes of pregnancy make both pregnant women and women who just gave birth more susceptible to VTE.
In clinical terms, the increased production of fibrin, decreased fibrinolytic activity and slowed blood flow from the enlarged uterus pressing on the inferior vena cava and pelvic veins lead to a hypercoaguable state in pregnancy. These changes put pregnant women at higher risk of blood clots.
At the same time, pregnancy brings a 20% to 25% increase in overall circulatory volume. The right iliac artery compresses the left iliac vein, which may explain the high incidence of DVT in the left leg during pregnancy.
These effects may be exacerbated when a pregnant woman lies on her back for prolonged periods, such as during bed rest, and blood flow velocity decreases. By 25 to 29 weeks of gestation, venous blood flow velocity in the legs decreases by almost half, a change that lasts until approximately six weeks after delivery.