Blood clots in the legs, known as deep vein thrombosis (DVT), form when blood flow slows down, a vein is damaged, or the blood itself becomes more prone to clotting. Between 1 and 3 out of every 1,000 adults develop one each year, and that number climbs to 2 to 7 per 1,000 in people over 70. Most leg clots result from a combination of factors rather than a single cause.
How a Clot Forms in a Vein
Three conditions set the stage for a blood clot: slow or stagnant blood flow, changes in the blood that make it clot more easily, and damage to the inner wall of a vein. When blood pools in the deep veins of the leg, especially in the pockets around the valves that keep blood moving upward, oxygen levels drop quickly. In animal and human studies, blood trapped in these valve pockets becomes oxygen-depleted within about two hours of stagnant flow, which triggers an inflammatory response and activates clotting proteins. The result is a clot that anchors itself to the vein wall and can grow over time.
Immobility and Prolonged Sitting
Anything that keeps your legs still for hours raises your risk. Your calf muscles act as pumps that squeeze blood back toward the heart with each step. When those pumps stop working, blood pools in the lower legs. This is why DVT is linked to long hospital stays, being bedridden for more than three days, and wearing a leg cast or brace.
Travel is a well-known trigger. The CDC considers any trip longer than four hours, whether by plane, car, bus, or train, a risk factor. The cramped seating on flights compounds the problem by compressing the veins behind the knees. Flexing your ankles, straightening your legs, and getting up to walk periodically all help keep blood circulating.
Surgery
Major surgery is one of the strongest single risk factors for leg clots. Operations on the abdomen, pelvis, hip, or legs carry the highest risk because they involve veins close to the surgical site and require prolonged time under anesthesia when muscles are completely relaxed. The danger doesn’t end at discharge: 60% of post-surgical blood clots develop after the patient has already left the hospital, sometimes up to 90 days later. Only about 40% form while the patient is still an inpatient. This is why surgeons often prescribe blood thinners or compression devices that extend well into recovery at home.
Hormonal Factors
Estrogen-containing birth control is a significant cause of clots in younger women who might otherwise have very low risk. When estrogen is taken by mouth, it passes through the liver first, which ramps up production of several clotting proteins while simultaneously lowering the body’s natural anticoagulants. The net effect is blood that clots more readily. Combined hormonal contraceptives roughly triple the risk of a venous clot overall, though the exact increase depends on the formulation. Some newer pill types and the patch carry a risk six to eight times higher than using no hormonal contraception at all.
Non-oral options like the patch and vaginal ring skip that first pass through the liver, but they still shift the blood’s clotting balance enough to raise risk. Progestin-only methods (certain IUDs, the mini-pill, the implant) carry a much lower clotting risk because they don’t contain estrogen.
Pregnancy and Postpartum
Pregnancy naturally increases clotting proteins to protect against hemorrhage during delivery, but that same shift makes clots more likely. The growing uterus also presses on the large veins in the pelvis, slowing blood return from the legs. Risk peaks in the first six weeks after delivery, when roughly 22 clot-related events (including clots in the legs, lungs, and clot-related strokes or heart attacks) occur per 100,000 deliveries. Elevated risk persists, at a lower level, through about 12 weeks postpartum.
Inherited Clotting Disorders
Some people are genetically predisposed to clot more easily. The two most common inherited mutations in people of European descent are Factor V Leiden, which affects about 1 in 20 people, and a prothrombin gene mutation, which affects about 1 in 50. Carrying one copy of the Factor V Leiden mutation roughly doubles clot risk (about 2.3 times higher than average), while one copy of the prothrombin mutation raises it by nearly twice normal (about 1.9 times).
About 1 in 1,000 people carry one copy of each mutation. In a large study of nearly 940,000 participants, these double carriers had about five times the risk of a venous clot compared to people without either mutation. That elevated risk held up even after accounting for body weight, blood type, and inflammation. Many carriers never know they have these mutations until a clot occurs, sometimes alongside another trigger like surgery, travel, or starting birth control.
Cancer
Cancer is both a cause of blood clots and, sometimes, the reason a clot is the first sign of an undiagnosed tumor. Many tumors release tiny particles into the bloodstream that carry a protein capable of kickstarting the clotting process. This is especially pronounced in pancreatic cancer and certain brain tumors, where production of this clotting trigger is particularly high. Lung and colorectal cancers can also drive clot formation through similar pathways, especially when specific gene mutations are present in the tumor itself.
On top of this direct effect, cancer patients face additional clot triggers: chemotherapy damages blood vessel walls, surgery is common, and patients often spend long stretches in bed. Active cancer is weighted heavily in the clinical scoring systems doctors use to assess clot probability.
Other Contributing Factors
Obesity increases clot risk by placing extra pressure on the veins in the pelvis and legs, slowing return blood flow. Age is a strong independent factor; the sharp rise in DVT rates after age 70 reflects cumulative vein wear, reduced mobility, and a higher likelihood of other medical conditions. Smoking damages vein walls and makes blood stickier. Heart failure and inflammatory diseases like lupus or inflammatory bowel disease also shift the balance toward clotting.
Previous DVT is one of the strongest predictors of a future clot. A first episode can damage the vein valves, making blood pool more easily in the same area long after the original clot resolves.
Recognizing Symptoms
A blood clot in the leg typically causes swelling in one leg (not both), pain or tenderness along the inner thigh or calf, warmth over the affected area, and sometimes redness or visible surface veins that weren’t there before. Clinicians look for measurable differences: a calf that’s more than 3 centimeters larger than the other side, pitting edema (where pressing a finger into swollen skin leaves an indent), and tenderness that follows the path of the deep veins. Some clots produce no obvious symptoms at all, which is part of what makes them dangerous.
Why It Matters: Pulmonary Embolism
The most serious consequence of a leg clot is that part of it breaks free and travels to the lungs. Untreated, this carries a mortality rate as high as 30%. With prompt treatment, that drops to about 8%. Sudden shortness of breath, chest pain that worsens with deep breathing, a rapid heart rate, or coughing up blood after having leg symptoms are warning signs that a clot may have reached the lungs.