Blood clots form when three conditions overlap: damage to a blood vessel wall, slow or stagnant blood flow, and blood that’s more prone to clotting than normal. You don’t need all three at once, but the more of these factors present, the higher your risk. Up to 900,000 people in the United States are affected by blood clots each year, and an estimated 60,000 to 100,000 die from them, making this one of the most common and serious vascular problems.
The Three Factors Behind Every Blood Clot
Your body forms clots as a survival mechanism. When you cut yourself, proteins and cell fragments rush to the site, build a mesh of fibrin, and seal the wound. Problems start when this process activates inside a blood vessel where there’s no injury to fix, or when a clot grows large enough to block blood flow.
Doctors have recognized since the 1800s that clotting comes down to three overlapping triggers: vessel wall damage, sluggish blood flow, and a hypercoagulable state (meaning the blood itself clots too easily). Most blood clots involve at least two of these factors working together.
Damage to Blood Vessel Walls
The inner lining of your blood vessels, called the endothelium, is more than just a tube. It actively releases substances that control blood pressure, manage inflammation, and decide when clotting should happen. When this lining gets damaged, it exposes underlying tissue that triggers the clotting process.
Common causes of vessel wall damage include chronically elevated blood pressure, smoking, high blood sugar, high cholesterol, and obesity. These don’t cause a single dramatic injury. Instead, they wear down the vessel lining over months and years, creating spots where clots are more likely to form. Surgery and physical trauma can also damage vessels directly, which is why blood clots are a major concern after operations.
In arteries specifically, damage often involves atherosclerosis, a buildup of fats, cholesterol, and other substances that form plaques on artery walls. These plaques narrow the vessel and can rupture. When a plaque bursts open, your body treats it like a wound and forms a clot on the spot. If that clot blocks blood flow to the heart, it causes a heart attack. If it blocks flow to the brain, it causes a stroke.
Slow or Stagnant Blood Flow
Blood that moves slowly or pools in one spot gives clotting proteins time to accumulate and stick together. This is why blood clots in veins, particularly the deep veins of the legs, are so closely tied to immobility. Veins have small valves that keep blood moving upward against gravity, and the pockets near these valves are common sites for clots to start forming when flow slows down.
Situations that slow blood flow enough to raise your clot risk include:
- Prolonged sitting or standing. Long car rides, flights, or desk work keep your leg muscles from pumping blood back toward the heart.
- Bed rest after surgery or illness. Even a few days of immobility significantly increases risk.
- Paralysis or limited mobility. Any condition that prevents regular leg movement reduces blood flow in the deep veins.
- Hospitalization. Blood clots are a leading cause of preventable death in hospitals, which is why patients often receive compression devices or blood thinners during stays.
On long flights or car rides, flexing and extending your legs, feet, and ankles every 30 minutes helps keep blood circulating and reduces pooling in the lower legs.
Blood That Clots Too Easily
Some people’s blood is chemically tilted toward clotting. This can be something you’re born with or something that develops later in life. Acquired causes are more common than inherited ones.
The most common inherited clotting disorder is Factor V Leiden, a genetic variation affecting 1% to 5% of the population. People with this condition have a higher risk of developing a first deep vein thrombosis (a clot in the deep veins, usually in the leg). The second most common inherited type is prothrombin thrombophilia, which affects a similar percentage of people and raises the risk of both deep vein clots and pulmonary embolism, where a clot travels to the lungs.
Acquired causes of hypercoagulability are more varied and more common overall. Cancer, diabetes, HIV, and certain heart conditions can all shift your blood chemistry toward easier clotting. The most common acquired clotting disorder is antiphospholipid syndrome, an autoimmune condition where the body produces antibodies that interfere with normal clotting regulation.
Hormones and Birth Control
Estrogen increases the liver’s production of clotting proteins, which is why hormonal factors are a significant and often underappreciated risk. Women who take estrogen-containing oral contraceptives have a 3 to 5 times higher risk of developing a blood clot compared to women who have never used them. The highest risk window is the first two years of use.
Pregnancy raises clot risk through a similar mechanism. The body naturally increases its clotting ability during pregnancy to prepare for the bleeding that comes with delivery. Combined with the pressure a growing uterus places on pelvic veins (slowing blood flow), pregnancy checks two of the three clotting triggers at once. The risk remains elevated for several weeks after delivery.
Surgery and Hospitalization
Major surgery is one of the strongest single risk factors for blood clots, especially procedures involving the abdomen, pelvis, hips, or legs. Surgery damages blood vessels directly, requires periods of immobility during recovery, and triggers an inflammatory response that can make blood clot more readily. All three clotting triggers activate at once.
Blood clots are the fifth most frequent reason for unplanned hospital readmissions after surgery overall. For patients undergoing total hip or knee replacement, they’re the third most frequent reason. This is why hospitals typically use prevention measures like compression sleeves on the legs and blood-thinning medications. After major surgery, clot prevention often extends beyond discharge, sometimes continuing for three to six weeks at home depending on the procedure and individual risk.
Arterial Clots vs. Venous Clots
Not all blood clots are the same. Where they form determines what they do and how dangerous they are.
Venous clots form in veins, most commonly in the deep veins of the legs. These are the clots associated with immobility, surgery, and hormonal risk factors. The primary danger is that a piece breaks off and travels to the lungs, becoming a pulmonary embolism. Sudden death is the first symptom in roughly 25% of people who have a pulmonary embolism, which is why prevention matters so much.
Arterial clots form in arteries and are more closely tied to atherosclerosis. When fatty plaques on artery walls rupture, a clot forms rapidly at the site. Because arteries carry oxygen-rich blood to organs, a blockage can quickly starve tissue. An arterial clot in a coronary artery causes a heart attack. One in a brain artery causes a stroke. The risk factors for arterial clots overlap with cardiovascular disease risk factors: high cholesterol, high blood pressure, smoking, diabetes, and obesity.
Warning Signs of a Blood Clot
A deep vein clot in the leg typically causes swelling, tenderness, redness, and warmth in the affected area. The symptoms usually appear in one leg, not both. Some clots cause no noticeable symptoms at all, which is part of what makes them dangerous.
If a clot travels to the lungs, symptoms shift to the chest: sudden shortness of breath, sharp chest pain that worsens with deep breathing, rapid heart rate, and sometimes coughing up blood. A clot in the lungs can lower blood oxygen levels, causing lightheadedness or fainting. These symptoms require emergency medical attention because pulmonary embolism can be fatal within hours.
Who Is Most at Risk
Risk factors compound. A person who smokes, takes estrogen-containing birth control, and sits at a desk all day has a meaningfully higher risk than someone with just one of those factors. The major risk factors, gathered together, include:
- Immobility from travel, bed rest, hospitalization, or sedentary work
- Recent surgery, particularly involving the hips, knees, pelvis, or abdomen
- Estrogen use from oral contraceptives or hormone replacement therapy
- Pregnancy and the postpartum period
- Cancer and cancer treatment
- Obesity
- Smoking
- Inherited clotting disorders like Factor V Leiden
- Age over 60, though clots can happen at any age
Understanding your personal combination of risk factors is the most practical thing you can do. Many of these, like immobility, smoking, and obesity, are modifiable. Others, like genetics or a cancer diagnosis, aren’t preventable but can be managed with awareness and, when appropriate, preventive blood thinners prescribed by a doctor.