Blood clots in the legs, known as deep vein thrombosis (DVT), form when three conditions overlap: blood flow slows down, the blood itself becomes more prone to clotting, or the inner lining of a vein gets damaged. Up to 900,000 people in the United States are affected each year, and an estimated 60,000 to 100,000 die from complications. Understanding the specific triggers helps you recognize whether you’re at higher risk.
How a Clot Forms Inside a Vein
Your blood is constantly balancing two jobs: flowing freely enough to circulate, and clotting quickly enough to seal a wound. A leg clot happens when that balance tips toward clotting in the wrong place. Three overlapping factors drive the process.
The first is slow blood flow. When blood pools in your leg veins instead of moving steadily back toward the heart, clotting proteins and activated blood cells accumulate in one spot rather than being swept away. This local buildup triggers a chain reaction that produces fibrin, a protein that forms sticky threads, trapping red blood cells and platelets into a solid mass.
The second is damage to the vein wall. Veins are lined with a smooth inner layer that normally prevents clotting. When that layer is injured, from surgery, a catheter, or blunt trauma, the exposed tissue releases powerful chemical signals that activate clotting almost immediately.
The third is blood that clots too easily. Certain conditions make your blood inherently “stickier,” whether from genetics, hormones, or chronic illness. When any two or all three of these factors are present at the same time, the risk of a leg clot rises sharply.
Immobility and Slow Blood Flow
Your calf muscles act as a pump, squeezing deep veins with every step to push blood upward against gravity. When you stop moving, that pump shuts off and blood pools in your lower legs. This is why prolonged immobility is one of the most common causes of DVT.
Being bedridden for more than three days, whether after surgery, during a hospital stay, or due to illness, significantly raises your risk. The same principle applies to leg casts or braces that prevent normal muscle contractions. Even sitting still for extended periods counts: the CDC notes that anyone traveling more than four hours by plane, car, bus, or train can be at risk for blood clots. It’s not the altitude or cabin pressure on a flight that matters most. It’s the hours spent with your legs bent and motionless.
People with paralysis or significant leg weakness face a persistent version of this problem, since the calf pump is permanently impaired on the affected side.
Surgery and Physical Trauma
Major surgery is one of the strongest triggers for leg clots because it can activate all three risk factors simultaneously. The operation damages blood vessel walls, anesthesia and bed rest slow circulation, and the body’s inflammatory response makes blood more prone to clotting.
Orthopedic procedures carry especially high risk. Without preventive treatment, the incidence of symptomatic DVT in the five weeks following major orthopedic surgery is roughly 4.3%. With standard blood-thinning prevention, that drops to about 1.5%. Hip and knee replacements, pelvic surgery, and major abdominal operations all fall into the high-risk category. This is why hospitals routinely use compression devices on your legs and prescribe blood thinners after these procedures.
Trauma to the leg itself, such as a fracture or severe muscle injury, can also damage veins directly and trigger local clot formation even without surgery.
Hormones and Birth Control
Estrogen increases the liver’s production of several clotting proteins, which is why hormonal factors play a significant role in DVT risk for women. Combined oral contraceptives (those containing both estrogen and progestin) raise the risk of blood clots by a factor of three to five compared to women who have never used them.
That elevated risk is highest during the first two years of use. A large study of 240,000 women in the UK Biobank found a hazard ratio of about 3.09 during the initial two years, meaning roughly triple the baseline risk. After that early window, the excess risk appears to level off substantially. Pregnancy and the postpartum period carry a similar hormonal shift, with clotting risk peaking in the weeks after delivery when estrogen levels are still elevated and mobility may be limited.
Hormone replacement therapy used during menopause also increases DVT risk, though typically less dramatically than oral contraceptives.
Genetic Clotting Disorders
Some people inherit gene variants that make their blood clot more aggressively than normal. The two most common are Factor V Leiden and a prothrombin gene mutation, both of which occur frequently in people diagnosed with venous blood clots. Factor V Leiden is particularly common in people of European descent, carried by roughly 5% of that population.
Having one copy of either mutation modestly raises your lifetime DVT risk. Having two copies, or carrying both mutations together, raises it much more. These genetic traits often go undetected until a clot happens, especially when a younger person develops DVT without an obvious trigger like surgery or immobility. If you’ve had an unexplained clot or have close family members who developed clots before age 50, genetic testing can identify whether an inherited condition is a factor.
Cancer and Chronic Illness
Active cancer is one of the strongest independent risk factors for DVT. Tumors release substances that activate clotting pathways, and many cancer treatments, including chemotherapy and surgery, compound the risk further. In clinical scoring systems used to assess DVT likelihood, a current cancer diagnosis adds significant weight to the evaluation.
Other chronic conditions also contribute. Heart failure slows blood return from the legs. Inflammatory bowel disease and autoimmune conditions create ongoing inflammation that promotes clotting. Obesity increases pressure in the veins of the pelvis and legs, slowing blood flow, while also raising levels of clotting factors in the blood. Severe infections and kidney disease can similarly shift the clotting balance.
Recognizing the Warning Signs
About half of all DVTs produce noticeable symptoms, while the rest are initially silent. When symptoms do appear, they typically affect one leg and include swelling (sometimes with the calf measuring more than 3 centimeters larger than the other side), pain or cramping that often starts in the calf, skin that looks red or purple depending on your natural skin tone, and a feeling of warmth over the affected area. The leg may also develop visible surface veins that weren’t there before, and pressing on the skin may leave a temporary dent, known as pitting edema.
These symptoms overlap with muscle strains and other common leg problems, which is why doctors use a structured scoring system that weighs your symptoms against your risk factors to determine how likely a clot is before ordering imaging.
Why Leg Clots Are Dangerous
The primary danger of a DVT is that part of the clot can break loose and travel through the bloodstream to the lungs, causing a pulmonary embolism (PE). Research from the American Heart Association found that when calf vein clots extend upward into larger leg veins, the risk of a detectable PE reaches 40% to 50%. Sudden death is the first symptom in about 25% of people who experience a pulmonary embolism, which is why DVT is treated urgently even when leg symptoms seem mild.
Even clots that don’t travel to the lungs can cause lasting damage. A condition called post-thrombotic syndrome develops in roughly a third of DVT patients, causing chronic leg swelling, pain, and skin changes that can persist for years after the original clot dissolves. The vein’s internal valves get damaged by the clot, permanently impairing blood flow in the affected leg.
Who Faces the Highest Risk
Risk factors stack. A person using oral contraceptives who also carries Factor V Leiden and takes a long flight faces a dramatically higher combined risk than someone with just one of those factors. The people most vulnerable to leg clots tend to have multiple overlapping triggers present at the same time.
- Recent surgery or hospitalization combined with bed rest
- Active cancer during or after treatment
- Hormonal contraceptive use paired with a genetic clotting disorder, smoking, or obesity
- Prolonged immobility from travel, injury, or illness in someone over 60
- Personal or family history of previous blood clots
Age itself is a factor. DVT risk roughly doubles with each decade after 40, as veins lose elasticity and blood flow naturally slows. But younger people are not immune, particularly when hormonal, genetic, or surgical risk factors are in play.