Pregnancy and the increased risk of developing blood clots

The three main risk factors that increase the risk of developing deep vein thrombosis and/or pulmonary embolism are abnormal clotting, reduced blood flow, and damage to the veins. These risks are all higher during pregnancy, most likely because of:

• Changes in hormone levels and blood composition that influence clotting.
• Reduced blood flow in the legs due to the weight of the fetus pressing upon veins.
• Injury to veins during delivery or surgery.
• Inactivity after cesarean section surgery or delivery.

Women who are obese, are older than 35, or have a family or personal history of blood clots have a higher risk of developing a clot that can lead to pulmonary embolism.

Soon after giving birth, the risk of developing deep vein thrombosis or pulmonary embolism rises by 5 times.¹ If a woman has a cesarean section, she is even more likely to develop one or more of these clots. This risk usually returns to normal after a few weeks after delivery.

Women with the following history may be screened for genetic factors that can increase the risk of forming blood clots:

• A personal or family history of deep vein thrombosis or pulmonary embolism
• Repeated miscarriages, especially during the second trimester
• Stillbirth
• Severe or recurrent low birth weight (intrauterine growth restriction)
• Preeclampsia

For pregnant women who are more likely to develop blood clots, several methods may be used to prevent deep vein thrombosis and pulmonary embolism. These include:

• Wearing compression stockings.
• Taking unfractionated heparin (UH) or low-molecular-weight heparin (LMWH).

For pregnant women who are diagnosed with deep vein thrombosis or pulmonary embolism, treatments may include:

• Heparin (an anticoagulant medication). Unfractionated heparin or low-molecular-weight heparin is used because neither has been shown to affect the fetus.
• Warfarin (another type of anticoagulant). This medication can be used after delivery. Warfarin may cause miscarriage or birth defects if used during pregnancy, especially in the first 6 to 9 weeks. There is also an increased risk of bleeding in the fetus and the mother, particularly during the third trimester.

If a woman has deep vein thrombosis during or after pregnancy, anticoagulant medication is usually continued for 6 weeks to 3 months after giving birth.²

Citations

1. Helt JA, et al. (2005). Trends in the incidence of venous thromboembolism during pregnancy or postpartum: A 30-year population study. Annals of Internal Medicine, 143(10): 697-706.

2. Kearon C, Hirsh J (2007). Venous thromboembolism. In DC Dale, DD Federman, eds., ACP Medicine, section 1, chap. 18. New York: WebMD.