Pregnancy and the Increased Risk of Developing Blood Clots

Topic Overview

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:

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

  • Heparin, an anticoagulant medicine. Heparin is used because it has not been shown to affect the fetus.
  • Warfarin, another type of anticoagulant. This medicine can be used after delivery. Warfarin may cause miscarriage or birth defects if used during pregnancy. 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 medicine is usually continued for 6 weeks to 6 months or more after giving birth.



  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.


Author Robin Parks, MS
Editor Kathleen M. Ariss, MS
Associate Editor Pat Truman, MATC
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Jeffrey S. Ginsberg, MD - Hematology
Last Updated January 5, 2010

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