Thrombosis in the legs is a blood clot that forms inside one of the deep veins, usually in the calf or thigh. The medical name is deep vein thrombosis, or DVT. It partially or fully blocks blood flow in the vein, causing swelling and pain in the affected leg. Left untreated, part of the clot can break loose and travel to the lungs, creating a potentially life-threatening emergency called a pulmonary embolism.
How a Clot Forms
Blood clots in the legs typically start at the tiny pockets inside venous valves, the one-way flaps that keep blood moving upward toward the heart. Three conditions drive clot formation: sluggish blood flow (stasis), damage to the inner lining of the vein, and blood that clots more easily than normal. These three factors, known collectively as the Virchow triad, rarely act alone. Most people who develop a leg clot have at least two of them working together.
The clots themselves are made mostly of a protein mesh called fibrin, along with red blood cells and small amounts of platelets. Once a clot takes hold, it can grow along the length of the vein, extending from the calf veins upward into the thigh or pelvis. That proximal spread is what raises the risk of a piece breaking off and reaching the lungs.
Common Symptoms
The hallmark sign is swelling in one leg, not both. The affected leg may feel warm to the touch, and the skin over the clot can turn red or appear flushed. Tenderness is present in about 75% of confirmed cases, typically concentrated in the calf muscles or along the inner thigh where the deep veins run. Some people describe a dull ache or tightness that worsens when standing or walking.
In severe cases where a large vein in the upper thigh or pelvis is completely blocked, the entire leg can become swollen, pale, and painful. This is sometimes called “painful white inflammation.” An even rarer presentation involves massive swelling with a bluish discoloration and compromised blood supply to the leg, which requires emergency treatment.
The tricky part is that DVT symptoms overlap with many other conditions. Calf tenderness alone shows up in roughly half of people who turn out not to have a clot. And some clots produce no noticeable symptoms at all, especially small ones confined to the calf veins.
Who Is Most at Risk
Anything that slows blood flow through the legs significantly raises your risk. The most common triggers include:
- Surgery: Major operations involving the abdomen, pelvis, hip, or legs carry the highest risk because of both tissue injury and prolonged bed rest afterward.
- Immobility: Being confined to bed during a hospital stay, wearing a leg cast, or sitting for long periods (including extended travel) all allow blood to pool in the lower legs.
- Hormonal factors: Estrogen-containing birth control pills, hormone replacement therapy, and pregnancy all increase clotting tendency. The elevated risk from pregnancy extends up to three months after giving birth.
- Inherited clotting disorders: Some people carry genetic mutations that make their blood clot more readily. A family history of DVT or pulmonary embolism is a meaningful warning sign.
- Paralysis: Loss of muscle movement in the legs removes the pumping action that normally pushes venous blood back toward the heart.
How DVT Is Diagnosed
Doctors don’t rely on symptoms alone because the physical signs are too unreliable. Instead, diagnosis follows a structured process. The first step is a clinical scoring tool called the Wells score, which assigns points based on risk factors and physical findings to estimate how likely a clot is before any testing begins.
If your probability score comes back low (“unlikely”), you’ll typically get a blood test called a D-dimer. This measures a protein fragment released when the body breaks down clots. The test is very sensitive, meaning a negative result is excellent at ruling DVT out. However, D-dimer levels rise with age, infection, surgery, and many other conditions, so a positive result doesn’t confirm a clot on its own.
If your probability score is higher, or the D-dimer comes back elevated, the next step is an ultrasound of the leg veins. The technician presses the ultrasound probe against each vein segment. A healthy vein collapses flat under pressure; a vein containing a clot won’t compress. This compression ultrasound is the standard imaging test for DVT and can pinpoint exactly where the clot sits. In some cases, a follow-up ultrasound is repeated five to seven days later to check whether a small calf clot has grown upward.
The Pulmonary Embolism Risk
The most dangerous complication of leg thrombosis is a pulmonary embolism, which occurs when a fragment of the clot travels through the bloodstream and lodges in the arteries of the lungs. Clots that remain isolated in the calf veins carry a relatively low risk of this, with studies showing embolism rates between 0% and 6.2% for calf-only clots. The danger increases substantially when clots extend into the thigh or pelvic veins, where the vessels are larger and clot fragments can be bigger.
Signs of a pulmonary embolism include sudden shortness of breath, chest pain that worsens with deep breathing, a rapid heart rate, and sometimes coughing up blood. This is a medical emergency.
Treatment
The cornerstone of DVT treatment is anticoagulation, medication that prevents the clot from growing and reduces the chance of embolism. Your body’s own clot-dissolving system then gradually breaks down the existing clot over weeks to months.
Newer oral anticoagulants have largely replaced the older approach of injectable blood thinners followed by warfarin. These newer drugs offer several practical advantages: they work within hours rather than days, require far less blood monitoring, and have fewer interactions with food and other medications. Warfarin, by contrast, requires regular blood tests to keep the dose in a narrow therapeutic range and interacts with a long list of foods and drugs. That said, warfarin remains the appropriate choice in certain situations, such as for patients with specific clotting disorders.
Treatment duration depends on what triggered the clot. A DVT caused by a clear temporary risk factor like surgery is often treated for three months. Clots that arise without an obvious trigger, or in people with ongoing risk factors, may require longer or even indefinite anticoagulation.
Post-Thrombotic Syndrome
Even after a clot dissolves, the affected vein may not return to normal. Between 20% and 50% of people who have had a DVT develop a condition called post-thrombotic syndrome (PTS), where lasting damage to the vein and its valves causes chronic problems in the leg. Five to ten percent of DVT patients progress to severe PTS.
Symptoms range from mild to debilitating. On the milder end, you might notice ongoing heaviness, aching, cramping, itching, or tingling in the leg, particularly after standing for long periods. More severe cases involve persistent swelling that doesn’t respond well to elevation, skin changes around the ankle, and in the worst scenarios, open sores (venous ulcers) on the lower leg that are slow to heal. Severe PTS can limit your ability to work and stay active. Wearing compression stockings after a DVT and staying physically active are the main strategies for reducing PTS risk.
Preventing Clots During Travel and Immobility
Long periods of sitting are one of the most common and preventable triggers. During flights or car trips, the CDC recommends getting up and walking every two to three hours. Choosing an aisle seat on a plane makes this easier. When you can’t stand, simple seated exercises make a real difference: raise and lower your heels while keeping your toes on the floor, then reverse the motion by lifting your toes while your heels stay down. Tightening and releasing your calf muscles also helps keep blood moving.
If you have additional risk factors, such as a recent surgery, a history of clots, or you’re taking estrogen-containing medication, talk to your doctor before a long trip about whether compression stockings or preventive medication makes sense. Aspirin is not recommended for travel-related clot prevention despite its reputation as a blood thinner; it works on a different part of the clotting process and hasn’t been shown to be effective for this purpose.