Blood clots in the leg, known as deep vein thrombosis (DVT), form when blood flow slows down, a vein wall gets damaged, or the blood itself becomes more prone to clotting. Usually, it takes a combination of these factors rather than a single cause. Understanding what triggers that combination can help you recognize whether you’re at higher risk.
The Three Conditions That Lead to a Clot
Every leg clot traces back to some mix of three factors: sluggish blood flow (stasis), damage to the inner lining of a vein, and blood that clots too easily (hypercoagulability). These three conditions, first described in the 1800s and still central to how doctors think about clotting, rarely act alone. A long flight slows blood flow in your legs, but a clot is far more likely to form if you’re also on hormonal birth control or recovering from surgery. The more of these factors present at once, the greater the risk.
Sitting Still for Too Long
Your calf muscles act as a pump, squeezing blood back up toward your heart every time you walk or shift your legs. When you sit or lie still for hours, that pump stops working and blood pools in the deep veins of your lower legs. This pooling, or stasis, is one of the most common triggers for DVT.
Long-distance travel is a well-known example. The CDC considers any trip lasting more than four hours a risk factor, while flights under four hours carry negligible risk for symptomatic clots. It’s not just air travel. Long car rides, train journeys, or any situation that keeps you seated with bent knees for hours can do the same thing. The edge of a seat pressing into the back of your knees can compress veins and worsen the problem. Bed rest after illness or injury works the same way: without regular movement, blood flow in the legs drops significantly.
Surgery and Physical Trauma
Major surgery, especially on the hip or knee, is one of the strongest triggers for leg clots. The operation itself damages blood vessels, the anesthesia and recovery period keep you immobile, and the body’s healing response makes blood clot more aggressively. Without preventive treatment, DVT develops in 22 to 33 percent of patients after hip replacement. That’s why hospitals routinely use blood thinners and compression devices after these procedures.
The risk window extends well beyond the operating room. Recent bed rest of three or more days, or any major surgery within the previous 12 weeks, is considered a meaningful risk factor. Broken bones, especially in the pelvis or legs, can damage nearby veins directly and keep you off your feet long enough for stasis to set in.
Hormonal Birth Control and Pregnancy
Combined hormonal contraceptives (pills, patches, or rings containing both estrogen and progestin) increase the blood’s tendency to clot. Compared to non-users, people taking these contraceptives have roughly 3.5 times the relative risk of developing a venous clot. In absolute terms, that translates to about 3 to 15 clot events per 10,000 users per year, compared to 1 to 5 per 10,000 among non-users. The risk is real but still small for most people. It rises further if you also smoke, carry extra weight, or have an inherited clotting condition.
Pregnancy shifts the balance even more. The body increases its clotting ability during pregnancy as a built-in defense against hemorrhage during delivery. At the same time, the growing uterus compresses the large veins in the pelvis, slowing blood return from the legs. The highest-risk period is actually just after delivery, and the elevated risk persists for about six weeks postpartum. For people with additional risk factors, doctors sometimes prescribe blood-thinning injections during this window.
Cancer and Other Medical Conditions
Cancer is one of the strongest medical risk factors for DVT. Tumors can compress veins, damage vessel walls, and release substances that make blood clot more readily. Certain cancers carry especially high risk. When adjusted for how common each cancer type is, pancreatic, ovarian, and brain cancers have the strongest association with blood clots. Chemotherapy can further raise the risk by damaging blood vessel linings and altering clotting proteins.
Other conditions that increase clot risk include heart failure (which slows circulation), inflammatory bowel disease, and severe infections. Any illness that keeps you in bed for days can contribute through immobility alone, but many of these conditions also push the blood toward a more clot-prone state through inflammation.
Inherited Clotting Disorders
Some people are born with blood that clots too easily. The two most common inherited causes are Factor V Leiden and a mutation in the prothrombin gene. These genetic variants are carried by a meaningful percentage of people with European ancestry, and many carriers go their entire lives without a clot. The mutations don’t guarantee a DVT, but they lower the threshold. Someone with Factor V Leiden who also takes hormonal birth control or has surgery faces a compounded risk that’s significantly higher than either factor alone.
If you’ve had a clot without an obvious trigger (no recent surgery, travel, or immobility), or if close family members have had clots at a young age, your doctor may test for these inherited conditions. Knowing your status can influence decisions about contraception, surgery preparation, and long-term prevention.
Other Risk Factors That Add Up
Age is a steady, compounding risk factor. DVT is uncommon in young, healthy adults but becomes increasingly common after 40, with risk roughly doubling each decade. Obesity slows venous return from the legs and increases baseline inflammation. Smoking damages blood vessel walls and makes blood stickier. A previous DVT is itself a risk factor for future clots, since the original clot can leave lasting damage to the vein’s valves and lining.
Paralysis or weakness in one leg, whether from a stroke, spinal cord injury, or a leg cast, eliminates the muscle-pump action on that side and creates the kind of prolonged stasis that strongly favors clot formation.
Recognizing the Warning Signs
DVT typically causes swelling in one leg (not both), often with pain or tenderness in the calf that feels like a deep cramp. The affected leg may feel warm to the touch, and the skin might look reddish or slightly discolored. Swelling that makes one calf more than 3 centimeters larger than the other is a clinically significant sign. You might also notice new, visible veins on the surface of the leg that weren’t there before.
Not every leg clot causes obvious symptoms, which is part of what makes DVT dangerous. The most serious complication is when a piece of the clot breaks loose and travels to the lungs, causing a pulmonary embolism. Sudden shortness of breath, chest pain that worsens with breathing, or coughing up blood are signs that need emergency attention. A leg clot itself is treatable and manageable, but it needs to be caught and treated with blood thinners before it has the chance to migrate.