Thrombophilia is a condition where your blood has an increased tendency to form clots. Sometimes called hypercoagulability, it happens when your body either produces too much of the proteins that trigger clotting or doesn’t produce enough of the proteins that prevent it. Many people carry thrombophilia their entire lives without knowing it, only discovering the condition after an unexpected blood clot or during testing for pregnancy complications.
How Normal Clotting Goes Wrong
Blood clotting is a survival mechanism. When you cut yourself, proteins in your blood activate in a chain reaction to form a plug and stop the bleeding. Equally important are the proteins that limit clotting, keeping it from spreading beyond where it’s needed. Thrombophilia disrupts this balance. Either the pro-clotting side is overactive or the anti-clotting side is too weak, and the result is the same: clots form when they shouldn’t, or grow larger than they should.
Three conditions make clotting more likely, a concept first described in the 1850s and still used today: blood that’s prone to coagulation, sluggish blood flow (stasis), and damage to blood vessel walls. Thrombophilia addresses the first of those three, but the others often act as triggers. A person with thrombophilia might go years without a clot, then develop one after a long flight or surgery because stasis or vessel injury tips the balance.
Inherited Thrombophilia
The most common form of inherited thrombophilia is the Factor V Leiden mutation, carried by 3 to 8 percent of people with European ancestry. About 1 in 5,000 people carry two copies of the mutation, which raises the risk considerably more than a single copy. This mutation makes one of the clotting proteins resistant to being switched off by the body’s natural braking system, leading to excess clot formation.
The second most common inherited type involves the prothrombin gene mutation (G20210A), which causes the body to produce higher than normal levels of prothrombin, a protein that drives clot formation. Both mutations increase the risk of clots in veins and, to a lesser extent, arteries.
Rarer inherited forms involve deficiencies in the body’s natural anticoagulant proteins: protein C, protein S, and antithrombin. These proteins normally act as a braking system, shutting down clotting once it’s done its job. When any of them is deficient, clotting can continue unchecked. Deep vein thrombosis in the legs accounts for roughly 90 percent of all clot events linked to these deficiencies.
Acquired Thrombophilia
Not all thrombophilia is inherited. Antiphospholipid syndrome is the most significant acquired form, an autoimmune condition where the immune system produces antibodies that interfere with normal clotting regulation. It can develop on its own or alongside other autoimmune diseases like lupus. Elevated homocysteine levels (hyperhomocysteinemia) are another acquired thrombophilic condition, sometimes linked to diet or vitamin deficiencies.
The distinction between inherited and acquired matters for treatment. Antiphospholipid syndrome, particularly the severe “triple positive” variant, requires a specific anticoagulation approach that differs from how genetic thrombophilia is managed.
What a Blood Clot Feels Like
Thrombophilia itself causes no symptoms. You feel nothing until a clot actually forms, and even then the signs can be subtle. A deep vein thrombosis (DVT), typically in the leg, causes pain or tenderness, swelling, warmth, and skin that looks red or discolored. It often affects just one leg.
The greater danger is pulmonary embolism (PE), which happens when a clot breaks free and travels to the lungs. Symptoms include sudden unexplained shortness of breath, sharp chest pain that worsens with breathing, coughing (sometimes with blood), and fainting. PE is a medical emergency. The CDC notes that the signs of both DVT and PE are nonspecific, meaning they overlap with many other conditions, which is one reason clots are sometimes missed.
Triggers That Raise the Risk
Having thrombophilia doesn’t guarantee you’ll develop a clot. Most clots happen when thrombophilia combines with an external trigger. The most common triggers include surgery (especially orthopedic procedures), prolonged immobility, and estrogen-containing medications.
The interaction between thrombophilia and hormonal contraceptives is striking. Women who carry the Factor V Leiden mutation and use estrogen-containing birth control pills face a 35- to 99-fold higher risk of venous blood clots compared to women without the mutation who don’t use oral contraceptives. For hormone replacement therapy combined with a clotting mutation, the risk is roughly 25 times higher than for women with neither risk factor. Obesity, diabetes, smoking, and polycystic ovary syndrome further compound these risks.
Pregnancy itself is an independent risk factor for clots, and the combination of pregnancy and thrombophilia raises VTE risk about 16-fold. This is actually more pronounced than the risk from oral contraceptives, which raises it about 2-fold in the same populations.
Thrombophilia and Pregnancy
Beyond blood clots, thrombophilia can affect the pregnancy itself. The placenta depends on healthy blood flow through small vessels, and a tendency toward excessive clotting can compromise that flow. Complications linked to thrombophilia include recurrent miscarriage, late pregnancy loss and stillbirth, preeclampsia (dangerously high blood pressure during pregnancy), placental abruption (where the placenta separates from the uterine wall early), and restricted fetal growth.
The relative risk of these complications is higher in women with thrombophilia, particularly for recurrent early miscarriage, late fetal loss, and preeclampsia. That said, the absolute risk remains small for most women. Many women with thrombophilia have uncomplicated pregnancies, especially when the condition is known and monitored.
How Thrombophilia Is Diagnosed
Diagnosis involves blood tests that measure clotting proteins, check for genetic mutations, and screen for antibodies associated with antiphospholipid syndrome. Timing matters significantly. Testing should not be done during an active clot or during the first three months of anticoagulant treatment, because both situations distort results and can produce false readings.
The recommended approach is to wait until at least three months of anticoagulant therapy have been completed, then stop the medication for two to four weeks before testing. This window also aligns with when other clot-related blood markers are commonly checked. Testing during an acute event or while on blood thinners is one of the most common causes of unreliable results.
Treatment and Long-Term Management
When a clot does occur, treatment centers on anticoagulant medications, commonly called blood thinners. Current guidelines favor direct-acting oral anticoagulants (DOACs) like apixaban or rivaroxaban as first-line options for most people with DVT or PE. These carry a lower risk of bleeding complications compared to the older approach of injectable blood thinners followed by warfarin. For people with triple positive antiphospholipid syndrome, warfarin remains the preferred choice because DOACs have not performed as well in that specific group.
Not everyone with thrombophilia needs lifelong medication. If you had a single clot with a clear trigger (like surgery), treatment might last three to six months. If clots are recurrent or unprovoked, long-term anticoagulation is more likely. The decision depends on the type of thrombophilia, the circumstances of the clot, and your individual risk profile.
Practical Steps to Lower Your Risk
If you know you have thrombophilia, daily habits can meaningfully reduce your clot risk. During long-distance travel, whether by air or car, get up and walk every two to three hours. Choose an aisle seat on flights so you can move freely. While seated, exercise your calves: raise and lower your heels with your toes on the floor, then raise and lower your toes with your heels on the floor. These small movements keep blood flowing through the deep veins of your legs.
Compression stockings can help during travel or prolonged sitting, particularly if you have additional risk factors. Staying well hydrated matters too, since dehydration thickens the blood and slows circulation. If you’re considering hormonal contraception or hormone replacement therapy, your thrombophilia status is a critical piece of information, because the combination can multiply your risk dramatically. Non-estrogen alternatives exist and are worth discussing with a prescriber who knows your clotting history.