Factor V Leiden is the most common inherited blood clotting disorder, caused by a single genetic mutation that makes your blood more likely to form clots than it should. About 5% of white Americans carry this mutation, though most will never develop a clot. The condition is named after the city of Leiden in the Netherlands, where researchers first identified it in the 1990s.
How the Mutation Works
Your blood has a built-in braking system. When a clot forms to stop bleeding, a protein called activated protein C (APC) eventually steps in to break the clot down so it doesn’t keep growing. It does this by cutting into another protein, factor V, at specific sites to deactivate it.
Factor V Leiden is a single-letter change in the gene that codes for factor V. That tiny change swaps one amino acid for another at exactly the spot where APC is supposed to cut. The result: APC can no longer properly shut down factor V. The clotting process keeps running longer than it should, which increases the chance that a clot forms when and where it shouldn’t, like deep inside a leg vein.
How Much It Raises Your Risk
Everyone inherits two copies of the factor V gene, one from each parent. If only one copy carries the Leiden mutation (heterozygous), your lifetime risk of developing a blood clot is roughly 7 times higher than someone without the mutation. If both copies carry it (homozygous), the risk jumps to about 20 times higher. Homozygosity is rare.
Those numbers sound alarming, but context matters. The baseline risk of a venous blood clot in the general population is low, roughly 1 to 2 per 1,000 people per year. A 7-fold increase on a small number is still a relatively small number. Most people with one copy of the mutation will go their entire lives without a clot. The risk becomes more significant when it stacks with other factors: surgery, immobility, pregnancy, or estrogen-containing medications.
Who Carries the Mutation
Factor V Leiden is overwhelmingly a European-ancestry mutation. In a large study of over 4,000 Americans, the carrier frequency was 5.27% in white Americans, 1.23% in African Americans, and just 0.45% in Asian Americans. If you have Northern European heritage, your odds of carrying the mutation are highest. In some Scandinavian populations, carrier rates run even higher than the U.S. average.
Symptoms to Watch For
Factor V Leiden itself causes no symptoms. You won’t feel anything different day to day. The mutation only becomes a problem if it contributes to an actual clot, most commonly a deep vein thrombosis (DVT) in the legs or a pulmonary embolism (PE) in the lungs.
Signs of DVT include pain or tenderness in one leg (often the calf), swelling, warmth in the affected area, and skin that looks red or discolored. A PE, which happens when a clot breaks loose and travels to the lungs, causes sudden shortness of breath, sharp chest pain that worsens with breathing, coughing (sometimes with blood), or fainting. A PE is a medical emergency.
How It’s Diagnosed
Two types of blood tests can identify Factor V Leiden. The first is a functional test that measures how well your blood responds to activated protein C. If your blood resists APC’s clot-slowing effects more than expected, it flags you as likely carrying the mutation. The second is a direct genetic test that looks at your DNA to confirm whether you have one or two copies of the mutated gene. The functional test is often used as a screening step, with genetic testing to confirm and determine whether you’re heterozygous or homozygous.
Testing is typically prompted by a clot that occurs at a young age, clots in unusual locations, a strong family history of blood clots, or recurrent clots without an obvious trigger. Routine screening of the general population isn’t standard practice.
Estrogen and Birth Control
Estrogen-containing contraceptives raise clotting risk on their own. Combined with Factor V Leiden, the risk multiplies rather than simply adding up. Studies comparing women who have the mutation and use oral contraceptives to women who have neither risk factor found the odds of a venous clot were 10 to 65 times higher, depending on the study. Even within the more conservative estimates, women with Factor V Leiden who used oral contraceptives had roughly 5 to 6.5 times the clot risk of women with the mutation who didn’t use hormonal birth control.
This doesn’t mean hormonal contraception is off the table entirely. Progestin-only options (pills, hormonal IUDs, implants) carry a much lower clotting risk than estrogen-containing pills, patches, or rings. If you know you carry Factor V Leiden, this is an important conversation to have when choosing contraception.
Pregnancy Considerations
Pregnancy naturally shifts the blood toward a more clot-prone state, which is why Factor V Leiden carriers face additional scrutiny during pregnancy. The concern isn’t just DVT. Researchers have investigated links between the mutation and recurrent pregnancy loss, preeclampsia, placental abruption, and restricted fetal growth.
The evidence, however, is mixed. A systematic review found that only about 37.5% of studies confirmed a clear relationship between Factor V Leiden and fetal loss, and no consistent association emerged for preeclampsia, placental abruption, or growth restriction. One Greek study did find the mutation in about 25% of women with a history of recurrent miscarriage, suggesting it may play a role in some cases. The bottom line is that carrying the mutation doesn’t guarantee pregnancy complications, but it does warrant closer monitoring, and some women may be prescribed preventive blood thinners during pregnancy and the postpartum period.
Treatment and Day-to-Day Management
If you carry Factor V Leiden but have never had a clot, you generally don’t need daily medication. The mutation alone isn’t treated with blood thinners. Management focuses on reducing the situational risks that could tip the balance toward a clot.
If you do develop a clot, treatment follows the same approach used for any venous thromboembolism: blood thinners (anticoagulants) for at least three months during the initial treatment phase. After that, your doctor evaluates whether to continue anticoagulation long-term based on factors like whether the clot was provoked by something temporary (surgery, immobility) or occurred spontaneously, and whether you’ve had more than one clot.
Practical Prevention for Carriers
The most actionable advice for Factor V Leiden carriers centers on high-risk situations. During long flights or car rides, get up and walk around periodically. Stay hydrated and avoid alcohol, which promotes dehydration and increases clotting risk. Compression stockings with moderate pressure (15 to 20 mmHg) can help prevent DVT on long trips.
Before any surgery or hospitalization, make sure your medical team knows about your mutation. Preventive blood thinners are commonly given to carriers during and after surgical procedures when mobility is limited. The same applies to any period of extended immobility, whether from an injury, illness, or bed rest. Staying active, maintaining a healthy weight, and being aware of the warning signs of DVT and PE are the most practical things you can do as a carrier living with this mutation.