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Factor V Leiden thrombophilia
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Factor V Leiden thrombophilia

Factor V Leiden thrombophilia is an inherited condition that increases the risk of developing abnormal blood clots due to a specific gene mutation.

Overview

Factor V Leiden thrombophilia is an inherited disorder of blood clotting. The condition is named after the specific gene mutation that causes thrombophilia, an increased tendency to form abnormal blood clots that can block blood vessels.

Symptoms & Risks

Individuals with this mutation have a higher-than-average risk of developing a deep venous thrombosis (DVT), which most commonly occurs in the legs. These clots can travel through the bloodstream and lodge in the lungs, a condition known as pulmonary embolism.

While the risk of clotting is increased, it is important to note that only about 10 percent of individuals with the mutation ever develop abnormal clots. The mutation is also associated with a slightly increased risk of pregnancy complications, including recurrent miscarriages, though most women with the condition have normal pregnancies.

Causes

The condition is caused by a mutation in the F5 gene, which provides instructions for making the protein coagulation factor V. In a healthy coagulation system, a protein called activated protein C (APC) inactivates factor V to prevent clots from growing too large. In people with Factor V Leiden, the factor V protein cannot be inactivated normally by APC, causing the clotting process to remain active longer than usual.

Other factors that increase clot risk include:

  • Increasing age
  • Obesity
  • Surgery or injury
  • Smoking
  • Pregnancy
  • Use of oral contraceptives or hormone replacement therapy containing estrogen

Inheritance

The risk of clotting depends on whether a person inherits one or two copies of the mutation. Inheriting two copies (one from each parent) carries a significantly higher risk of developing a blood clot compared to inheriting a single copy.

Frequently Asked Questions

The combination of estrogen-containing medications and the Factor V Leiden mutation significantly multiplies the risk of venous thromboembolism. For individuals with one copy of the mutation, the risk is already elevated; adding estrogen can increase the likelihood of a clot by several-fold compared to those without the mutation. Clinicians typically recommend avoiding estrogen-based contraceptives and hormone replacement therapy in favor of progestin-only methods or non-hormonal options like copper IUDs to minimize this compounded risk.
While you cannot change your genetic status, you can mitigate environmental triggers that exacerbate clotting risk. Key strategies include maintaining a healthy weight to reduce pressure on leg veins, staying physically active to promote circulation, and avoiding prolonged periods of immobility such as long car or plane rides. Additionally, quitting smoking is critical because smoking damages blood vessel walls and increases clotting factors. Staying well-hydrated is also recommended, especially during travel or hot weather, to prevent blood from becoming too viscous.
No, most women with Factor V Leiden have normal, healthy pregnancies. While the mutation is associated with a slightly increased risk of complications such as preeclampsia, placental abruption, or recurrent miscarriage, the majority of pregnancies proceed without issue. Your healthcare provider will likely monitor your pregnancy more closely and may discuss the use of low-dose aspirin or blood thinners during and after pregnancy if your personal history or risk profile warrants it, but this is not required for every patient.
Diagnosis is confirmed through a simple blood test that detects the specific genetic mutation in the F5 gene or measures the resistance of factor V to activated protein C. Testing is generally not recommended for asymptomatic individuals without a personal or family history of unexplained blood clots, as the results may cause unnecessary anxiety and do not always change management. However, if you have a strong family history of DVT or pulmonary embolism, or if you are planning to start estrogen-based hormone therapy, a clinician may recommend genetic testing to guide preventive care decisions.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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