Blood clots form in your legs when blood flow slows down, a vein wall gets damaged, or your blood becomes more prone to clotting. Often, it’s a combination of two or all three of these factors working together. The medical term is deep vein thrombosis (DVT), and up to 900,000 people in the United States are affected by clot-related events each year. Understanding what triggers clot formation helps you recognize whether you’re at higher risk.
The Three Conditions That Cause Clots
Every leg clot traces back to one or more of three conditions: sluggish blood flow (stasis), damage to the inner lining of a vein, and blood that clots too easily (hypercoagulability). These three factors interact dynamically. You might have slow blood flow from sitting for hours on a plane, but your body handles it fine. Add a genetic tendency toward clotting, and that same flight becomes genuinely dangerous.
Stasis is the most common trigger in everyday life. Your calf muscles act as a pump, squeezing blood upward through your veins with every step. When you stop moving for extended periods, blood pools in the deep veins of your lower legs, giving clots time to form. Vein damage can come from surgery, injury, or inflammation, and it activates the body’s clotting system at that site. Hypercoagulability, the third factor, can be inherited or caused by medications, pregnancy, or illness.
Sitting Still for Too Long
Prolonged immobility is one of the most common and preventable causes of leg clots. Any travel lasting more than four hours, whether by plane, car, bus, or train, raises your risk. It’s not something unique to air travel. The key factor is keeping your legs in the same position without engaging your calf muscles.
The same principle applies to bed rest after surgery or illness, desk jobs that keep you seated for hours, and long gaming or binge-watching sessions. Your veins rely on muscle contractions to push blood back toward your heart. Without that mechanical pump, gravity wins, and blood stagnates in the deep veins behind your knee and in your calf.
Surgery, Especially Joint Replacements
Surgery is one of the strongest risk factors for leg clots. The combination of tissue damage (which activates clotting), general anesthesia (which relaxes your veins and slows flow), and post-operative immobility creates near-perfect conditions for a clot. Orthopedic procedures carry particularly high risk: without preventive measures, the incidence of clot-related events after orthopedic surgery can reach as high as 60%.
Hip and knee replacements are the most studied examples, but any major surgery that keeps you off your feet for days increases risk. This is why hospitals routinely use blood-thinning medications and compression devices after operations. Graduated compression stockings alone reduce the incidence of post-surgical clots by roughly 60%, and when combined with other preventive methods, that reduction can reach 85%.
Hormonal Factors
Estrogen-containing medications significantly increase clot risk. Women taking combined oral contraceptives face a three- to five-fold higher risk of blood clots compared to women who have never used them. That elevated risk is highest during the first two years of use, when the hazard ratio peaks around 3.09, then drops back to near-baseline with continued use. Hormone replacement therapy during menopause carries a similar, though somewhat lower, increase in risk.
Pregnancy itself raises clot risk because estrogen levels surge, blood volume increases, and the growing uterus compresses the veins that drain the legs. The risk is highest in the third trimester and the first six weeks after delivery. Women who are both pregnant and carry certain genetic mutations face a compounded risk that’s considerably higher than either factor alone.
Genetic Clotting Disorders
Some people are born with blood that clots more readily than normal. The most common inherited clotting disorder is Factor V Leiden, a gene mutation carried by about 5% of people of European descent. Inheriting one copy of this mutation from a parent increases your chance of developing a clot by four to eight times. Two copies, one from each parent, can raise the risk up to 80-fold.
To put that in practical terms: the average person’s annual risk of a blood clot is about 1 in 1,000. One copy of Factor V Leiden shifts that to roughly 1 in 125 to 1 in 250. Two copies push it as high as 1 in 12. The real danger comes when genetic risk stacks with other factors. A woman with Factor V Leiden who takes oral contraceptives has about 35 times the usual risk of developing a clot, compared to a woman without the mutation who doesn’t take hormonal birth control.
Most people with clotting disorders don’t know they have one until a clot occurs, especially if they’ve never had a triggering event like surgery or prolonged immobility. If you’ve had a clot before age 40, clots in unusual locations, or a strong family history of clotting, genetic testing can help clarify your risk.
Cancer and Other Medical Conditions
Cancer is a well-established risk factor for blood clots. Tumors release substances that activate the clotting system, and cancer treatments like chemotherapy can damage blood vessel walls and reduce mobility. Some cancers carry particularly high clot risk: pancreatic, stomach, brain, lung, uterine, ovarian, and kidney cancers, along with blood cancers like lymphoma and myeloma.
Other medical conditions that increase risk include heart failure (which slows blood circulation), inflammatory bowel disease, obesity, and varicose veins. Obesity contributes through multiple pathways: it increases pressure on the veins in the pelvis and legs, raises levels of clotting factors in the blood, and promotes chronic low-grade inflammation.
What a Leg Clot Feels Like
A DVT typically causes swelling in one leg, not both. The affected leg may feel warm to the touch, and the skin might turn red or develop a bluish tint. Pain often starts in the calf and can feel like a cramp or deep soreness that doesn’t go away with stretching. Some people notice that the pain worsens when they flex their foot upward.
Not all DVTs cause obvious symptoms. Some are “silent” and discovered incidentally during imaging for another reason. This makes risk awareness important, because the first sign of a clot can sometimes be its most dangerous complication: a pulmonary embolism, which happens when a clot breaks free and travels to the lungs. Sudden shortness of breath, chest pain that worsens with breathing, rapid heartbeat, or coughing up blood are signs that require emergency care.
How Clots Are Detected
If your symptoms and risk factors suggest a clot, the first step is usually a blood test that measures D-dimer, a protein fragment produced when a clot dissolves. This test is highly sensitive, catching up to 97% of clots, but it’s not very specific (as low as 35% specificity). A normal D-dimer result is good at ruling out a clot. An elevated result doesn’t confirm one, so it’s typically followed by an ultrasound of the leg veins, which can directly visualize the clot and pinpoint its location.
Long-Term Effects of a Leg Clot
Even after a clot is treated and dissolved, it can leave lasting damage. Between 20% and 50% of people who have a DVT develop post-thrombotic syndrome, a chronic condition caused by damage to the vein’s internal valves. Symptoms include persistent swelling, aching, heaviness, and skin changes in the affected leg. About 5% to 10% of DVT patients develop a severe form that includes chronic skin ulcers near the ankle.
Post-thrombotic syndrome tends to develop within the first one to two years after a clot and can be a lifelong issue. This is one reason prevention matters so much. Wearing compression stockings after a DVT, staying active, and maintaining a healthy weight can reduce the severity of these long-term effects.
Reducing Your Risk
If you’re traveling for more than four hours, get up and walk around every hour or two. When you can’t stand, flex and extend your ankles repeatedly to engage your calf muscles. Staying hydrated helps keep your blood from thickening, and avoiding alcohol during travel is a practical step since it promotes dehydration.
After surgery, follow your care team’s instructions about getting up and moving as soon as it’s safe. Compression stockings are effective and low-risk. If you’ve been prescribed a blood thinner after a procedure, take it as directed, even if you feel fine and think you don’t need it.
For everyday life, the biggest modifiable risk factors are prolonged sitting, obesity, and smoking. Regular physical activity keeps blood circulating, helps maintain a healthy weight, and reduces inflammation. If you take estrogen-containing birth control or hormone therapy and have a family history of clotting, it’s worth discussing your personal risk profile with your provider to weigh the options.