Deep vein thrombosis (DVT) in the leg happens when a blood clot forms in one of the deep veins, typically in the calf or thigh. Three conditions drive clot formation: sluggish blood flow, damage to the vein wall, and blood that clots too easily. Most cases involve at least two of these factors occurring together, and understanding which ones apply to you helps explain why a clot formed or why your risk may be elevated.
How Blood Clots Form in Leg Veins
Your veins have a smooth inner lining that normally keeps blood flowing freely and prevents platelets from sticking together. When that lining is damaged, whether from surgery, injury, or inflammation, the body’s clotting system activates at the site. Platelets rush to the damaged area, and proteins in the blood begin forming a mesh of fibrin that traps red blood cells into a solid clot.
Leg veins are especially vulnerable because of how they’re built. The deep veins in your calves and thighs contain small valves that keep blood moving upward toward the heart. Blood can pool in the pockets next to these valves when you’re not moving, creating zones of stagnant flow. When blood sits still in these pockets, oxygen levels drop. That low-oxygen environment switches on clotting signals on the vein wall, attracting immune cells that carry clotting factors to the surface. This is why immobility is one of the strongest triggers for DVT.
The third piece is blood chemistry. Some people’s blood is naturally more prone to clotting because of genetic traits, hormones, or other medical conditions. When clot-prone blood meets slow flow and even minor vein damage, a DVT can develop within hours.
Immobility and Prolonged Sitting
Sitting or lying still for extended periods is one of the most common DVT triggers. Your calf muscles act as a pump, squeezing the deep veins to push blood upward each time you walk or flex your feet. When that pump stops working, blood pools in the lower legs. The CDC identifies any travel lasting more than four hours, whether by plane, car, bus, or train, as a period of increased risk. The longer you stay immobile, the greater the danger.
This same mechanism applies to hospital bed rest, recovery from surgery, or even long stretches at a desk. It’s not the airplane cabin or the car seat itself that creates the problem. It’s the stillness.
Surgery and Physical Trauma
Surgery, particularly on the hip, knee, or pelvis, is a major DVT risk factor because it combines all three clotting triggers at once: the operation damages blood vessels, anesthesia and recovery keep you immobile, and your body ramps up its clotting response to control bleeding.
Fractures of the leg carry significant risk on their own. In a large multicenter study, DVT was found at hospital admission in roughly 28-32% of patients with femoral shaft, hip, or pelvic fractures, and in about 12-14% of those with ankle, shin, or knee fractures. High-energy injuries like pelvic fractures cause direct damage to the vein walls, while hip fractures tend to occur in older adults whose age adds additional risk.
Genetic Clotting Disorders
Some people inherit genes that make their blood clot more aggressively than normal. The most common inherited clotting condition, called Factor V Leiden, is present in 15-20% of people experiencing their first DVT. It’s found in up to 50% of those who have recurrent clots or develop a clot while taking estrogen. A second common mutation, in the prothrombin gene, similarly increases clot risk.
Rarer inherited conditions involve deficiencies in natural anticoagulant proteins (protein C, protein S, and antithrombin). These affect less than 1-2% of the general population but are found in 1-3% of people who develop a first clot. If you’ve had a DVT at a young age, or if blood clots run in your family, a genetic clotting disorder may be a contributing factor.
Hormones, Pregnancy, and Birth Control
Estrogen increases the blood’s tendency to clot, which is why hormonal factors are a significant DVT trigger for women. Pregnancy raises DVT risk six to tenfold compared to non-pregnant women of the same age. The effect comes from rising estrogen and progesterone levels, increased blood volume, and the pressure the growing uterus places on pelvic veins, all of which slow blood return from the legs.
Estrogen-containing birth control pills carry a three to sixfold increase in relative risk. Hormone replacement therapy used for menopause symptoms raises the risk two to fourfold. These numbers represent relative risk, meaning the comparison is against women not using hormones. The absolute chance of a clot is still low for most women, but it rises sharply when combined with other factors like a genetic clotting disorder, obesity, or smoking.
Cancer and Chemotherapy
Cancer significantly increases the likelihood of DVT through several pathways. Many tumors release substances that activate the clotting system directly. Cancer cells can shed molecules that attract immune cells and platelets, promoting clot formation even at sites far from the tumor itself. Certain genetic mutations common in cancers, particularly colorectal cancers, cause tumor cells to produce high levels of a protein that triggers the clotting cascade.
Chemotherapy compounds the problem. Some treatments, including platinum-based drugs, further increase the rate of clotting events. Cancer patients are also more likely to be hospitalized, immobile, or undergoing surgery, layering additional risk factors on top of the biological changes caused by the disease itself. A DVT that appears without an obvious cause is sometimes the first sign of an undiagnosed cancer.
Obesity
Carrying excess weight puts continuous pressure on the veins in the pelvis and legs, slowing blood return to the heart. Obesity also creates a low-grade inflammatory state that shifts blood chemistry toward easier clotting. People with a BMI of 30 or higher have roughly two to five times the risk of developing a venous clot compared to those at a normal weight. One large analysis of nearly 20,000 patients found that obesity alone more than doubled the odds of clotting, and the risk climbs further at higher BMI levels.
Age
DVT risk increases dramatically with age. Among young adults, the annual incidence is about 1 in 10,000. In people over 60, it rises to roughly 1 in 100. Aging contributes through multiple channels: veins lose elasticity, valve function declines, physical activity tends to decrease, and chronic conditions that promote clotting become more common. Age alone doesn’t cause DVT, but it amplifies nearly every other risk factor on this list.
An Anatomical Cause: May-Thurner Syndrome
Some people have a structural issue that makes DVT more likely in the left leg specifically. In May-Thurner syndrome, the right iliac artery (which carries blood from the heart to the right leg) crosses over and compresses the left iliac vein against the spine. This compression restricts blood flow out of the left leg and can eventually cause internal scarring, called venous spurs, inside the vein.
The condition progresses through three stages: asymptomatic compression, spur formation, and finally DVT. The typical presentation is a younger woman who develops sudden swelling in the left leg, often triggered by an additional factor like surgery, pregnancy, or a period of immobility. May-Thurner syndrome is likely underdiagnosed because the compression itself doesn’t cause symptoms until a clot forms.
COVID-19 and Viral Infections
COVID-19 significantly raises the risk of blood clots, and the danger persists well beyond the acute illness. A CDC study found that people who had COVID-19 had a 63% higher rate of clotting events in the year after infection compared to people who had other respiratory illnesses. Even those who were never hospitalized had a 73% increased risk.
The virus appears to damage the inner lining of blood vessels, trigger an exaggerated immune response, and promote the formation of tiny, persistent microclots in the bloodstream. While the risk decreases over time after infection, it remains elevated for months. Other respiratory viruses, including influenza, also raise clotting risk, but COVID-19 does so to a greater degree.
When Multiple Factors Combine
DVT rarely results from a single cause acting alone. A woman on hormonal birth control who takes a long flight has two risk factors working simultaneously. A 65-year-old recovering from hip surgery has age, immobility, and surgical trauma converging. The interaction between factors is what makes DVT unpredictable: one person can sit through a 12-hour flight with no problems, while another develops a clot during a 5-hour car ride because they also have an undiagnosed clotting disorder.
Understanding your personal combination of risk factors is what matters most. Some, like genetics and age, you can’t change. Others, like staying mobile during travel, maintaining a healthy weight, and being aware of hormonal risks, give you something concrete to work with.