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Factor V Leiden and Pregnancy

The increased risk for blood clots caused by pregnancy combined with the increased risk for blood clots caused by Factor V Leiden should be taken very seriously.

It is fairly well known that the chemical changes caused by pregnancy create an increased risk for the development of dangerous blood clots. Women who have just given birth are also at an increased risk for blood clots. Pregnancy does not directly cause blood clots, but it does pose a four-fold increased risk for the development of a blood clot. That risk actually increases to about 20-fold in the weeks immediately following childbirth, and is at its highest — a risk of 100-fold — in the first week after the baby is born.


These blood clots usually occur in the veins and are known as a venous thrombosis (VTE). The most common type of VTE is called a deep vein thrombosis or DVT that usually develops in the deep veins of the legs. A portion of the DVT can break loose and travel to the lungs. This is known as a pulmonary embolism or PE and can become life-threatening. These are the most common types although abnormal blood clots can form anywhere including the brain.


This tendency for a woman’s body to form clots during pregnancy is the result of a natural biological response intended to protect women against the possibility of major bleeding challenges of miscarriage and childbirth.



An inherited blood clotting disorder or thrombophilia like Factor V Leiden further increase the risk for blood clots during pregnancy.

What are the risks of getting a venous blood clot during pregnancy if I have Factor V Leiden?

As mentioned, healthy women who become pregnant are estimated to have a 4-fold increased risk of developing a blood clot during their pregnancy.


Some studies have shown that women who have heterozygous Factor V Leiden (carry only one mutated factor V gene) have an 8 to 52-fold increase depending on coexisting risk factors such as obesity or advanced age. Homozygous carriers (they carry two mutated factor V genes) have much higher risks with some studies claiming as high as 100-fold.


Studies also show that women with multiple blood clotting disorders such as Factor V Leiden combined with the prothrombin 20210G>A mutation or other thrombophilia have the highest risk for pregnancy-associated VTE.


Although Factor V Leiden increases the relative risk for VTE during pregnancy and postpartum, the absolute risk in asymptomatic heterozygotes is not well defined. The available evidence suggests that the absolute incidence of thrombosis during pregnancy is low. 


In July, 2018, The ObG Project, an educational resource for Women’s Health, in association with the American College of Obstetricians and Gynecologists (ACOG), estimated the increased risks for blood clots during pregnancy related to Factor V Leiden:


  • Heterozygous type: VTE risk during pregnancy
    • Without a personal history of VTE or an affected first-degree relative under age 50:
      • Slight increased risk (5 – 12 / 1,000 deliveries)  
    • Affected first-degree relative but no personal history of VTE:
      • Slight increased risk (15 / 1,000 deliveries)  
    • Personal history of VTE:
      • 10% risk
  • Homozygous type: VTE risk during pregnancy
    • Without a personal history of VTE or an affected first-degree relative under age 50:
      • 1–2% risk 
    • Personal history of VTE or an affected first-degree relative under age 50:
      • 17% risk

How can I reduce my risk?

Women should discuss their potential risk factors with their healthcare professional or genetic counselor and make sure they take steps to address any risks they might identify. This should include a discussion about family history and any genetic blood clotting disorders that are known.


Treatment options have expanded with the development of several new anticoagulants (blood thinners) in recent years that can safely be taken during pregnancy and after the birth.

Where can I find more information?

Soon you will be able to visit our Research and Resource Library to obtain very in-depth information about Factor V Leiden and how it relates to pregnancy and postpartum. We will share with you the latest research and recommendations from the best professionals in their field.


In the meantime, the National Blood Clot Alliance (NBCA), working with the Alexandra L. Rowan Memorial Foundation, has put together some excellent information on pregnancy, anticoagulants, and the prevention of blood clots. To visit their website, click here.

To Learn More About Women & Blood Clots...

Each year, up to 900,000 people are affected by blood clots, and about 100,000 people will die because of blood clots. However, many of these deaths can be prevented simply by sharing life-saving information. To learn more about women and blood clot risks, signs and symptoms, and prevention, please visit and share

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