Blood clots in the leg, known as deep vein thrombosis (DVT), form when three conditions converge: blood flow slows down, the blood becomes more prone to clotting, or the wall of a vein is damaged. Most leg clots result from a combination of these factors rather than a single cause. About 1 in 1,000 people develop one each year, but certain situations can push that risk dramatically higher.
The Three Conditions That Trigger a Clot
Every leg clot traces back to one or more of three core problems: sluggish blood flow, blood that clots too easily, or damage to the inner lining of a vein. When blood pools in the deep veins of your calf or thigh instead of circulating normally, proteins in the blood have more time to stick together and form a solid mass. When the blood itself is chemically shifted toward clotting, even normal flow may not prevent one. And when the smooth inner surface of a vein is injured, by trauma, surgery, or inflammation, the body’s repair response can overshoot and produce a clot that partially or fully blocks the vessel.
In practice, these three problems overlap constantly. A person recovering from hip surgery, for example, has vein damage from the procedure, reduced blood flow from lying in bed, and an inflammatory state that makes their blood clot more aggressively. That triple hit explains why surgery is one of the strongest triggers.
Surgery and Immobility
Major surgery is one of the most well-documented causes of leg clots. Hip fracture repair carries a clot rate of about 1.6%, and colorectal cancer surgery runs around 1.67%. Compared to minor procedures, major orthopedic surgeries like knee and hip replacements carry a 20- to 34-fold higher risk. The combination of tissue damage, general anesthesia reducing muscle contractions in the legs, and days of limited movement creates ideal conditions for a clot to form.
You don’t need surgery to be at risk from immobility alone. Any situation that keeps your legs still for hours reduces the pumping action of your calf muscles, which normally helps push blood back toward your heart. The CDC notes that anyone traveling more than four hours, whether by plane, car, bus, or train, faces increased clot risk. Hospital bed rest, a leg cast, and even prolonged desk work without breaks can have a similar effect.
Hormones and Birth Control
Estrogen increases the liver’s production of clotting proteins, which is why hormonal birth control containing estrogen is a recognized risk factor. A large meta-analysis of 23 studies found that combined oral contraceptives raise the risk of a venous blood clot roughly 3- to 4-fold compared to not using them. The risk varies by formulation. Pills containing older progestins like levonorgestrel carry a roughly 2.8-fold increase, while newer progestins like desogestrel push the risk slightly higher, to about 3.8-fold.
Pregnancy raises clot risk through a similar hormonal mechanism, compounded by the growing uterus pressing on pelvic veins and slowing blood return from the legs. Hormone replacement therapy used during menopause also contains estrogen and carries an elevated risk, though typically lower than that of oral contraceptives in younger women.
Inherited Clotting Disorders
Some people are born with blood that clots more easily than normal. The most common genetic cause is a mutation called Factor V Leiden, which makes one of the proteins involved in clotting resistant to being switched off. Carrying one copy of this mutation raises the annual clot risk from about 1 in 1,000 to 3 to 8 in 1,000. Carrying two copies can push the risk as high as 80 in 1,000, a dramatic increase.
Factor V Leiden is most common in people of European descent. Other inherited mutations affect different parts of the clotting cascade and can compound the risk when they occur together. If you have a close relative who developed a blood clot before age 50, or who had clots without an obvious trigger, there may be a genetic component worth investigating. Having one of these mutations doesn’t guarantee a clot, but it lowers the threshold. Add a triggering event like surgery, pregnancy, or a long flight, and the combination can tip the balance.
Cancer
Cancer is one of the more serious underlying causes of leg clots, and in some cases a DVT is actually the first sign of an undiagnosed malignancy. Tumors promote clotting through several pathways at once. Many cancer cells release tiny membrane fragments into the bloodstream that provide a surface where clotting reactions can assemble. Pancreatic tumors are particularly aggressive in this regard, releasing fragments loaded with a protein that directly activates the clotting cascade. Brain tumors can release different fragments that activate platelets. Gastrointestinal, lung, breast, and ovarian cancers often cause elevated platelet counts, further lowering the threshold for clot formation.
Cancer also triggers the immune system in ways that promote clotting. White blood cells called neutrophils can release web-like structures into the bloodstream that trap clotting factors and accelerate thrombus formation. Chemotherapy itself adds to the risk by damaging blood vessel walls and altering blood chemistry. Patients with cancer undergoing surgery face a compounded risk from both the malignancy and the procedure.
Other Medical and Lifestyle Risk Factors
Several other conditions and habits contribute to leg clot formation:
- Obesity increases pressure on the veins in your pelvis and legs, slowing blood return and promoting clot-friendly stagnation.
- Smoking damages the inner lining of blood vessels and shifts blood chemistry toward clotting.
- Age is a consistent risk factor. Clot incidence rises steadily after age 40, with the highest rates in people over 70.
- Previous DVT significantly raises the chance of another. Once a vein has been damaged by a clot, the scarred vessel wall is more prone to forming new ones.
- Heart failure and inflammatory diseases like lupus or inflammatory bowel disease create chronic states that make blood more prone to clotting.
These risks are cumulative. A 65-year-old smoker with obesity who takes a transatlantic flight faces a very different level of risk than a healthy 30-year-old on the same flight.
What a Leg Clot Feels Like
The classic symptoms are swelling in one leg (not both), pain or cramping that often starts in the calf, warmth over the affected area, and skin that turns red or purple. The swelling typically develops over hours to days and may make one leg noticeably larger than the other. The pain often feels like a deep soreness or tightness rather than a sharp surface pain, and it can worsen when you stand or walk.
Not all leg clots announce themselves. DVT can occur without noticeable symptoms, especially when the clot is small or located in the calf veins rather than the larger thigh veins. This is part of what makes the condition dangerous: a clot you don’t feel can still break loose and travel.
Why Leg Clots Are Dangerous
Most DVTs start in the calf veins. About 10% to 20% of those extend upward into the larger veins of the thigh, and 1% to 5% eventually send a fragment into the lungs, causing a pulmonary embolism. A pulmonary embolism can be life-threatening, blocking blood flow through the lungs and straining the heart. Symptoms include sudden shortness of breath, chest pain that worsens with breathing, rapid heartbeat, and lightheadedness.
Even clots that don’t travel can cause lasting damage. The vein where the clot formed may develop chronic scarring and valve damage, leading to long-term swelling, pain, and skin changes in the affected leg. This is called post-thrombotic syndrome, and it affects a significant portion of DVT survivors.
How Leg Clots Are Detected
If a clot is suspected, the first step is often a blood test that measures a substance called D-dimer, a byproduct of clot breakdown. This test is highly sensitive (up to 97%), meaning a negative result is very good at ruling out a clot. But it has poor specificity (as low as 35%), so a positive result doesn’t confirm one. Many conditions, including recent surgery, infection, and pregnancy, can elevate D-dimer levels without a clot being present. When D-dimer is elevated or clinical suspicion is high, an ultrasound of the leg veins is the standard next step to visualize the clot directly.