Blood clots form when three conditions come together: damage to a blood vessel wall, sluggish blood flow, and blood that’s in a state primed for clotting. You can develop a clot from surgery, long periods of sitting still, certain medications, obesity, cancer, or an inherited tendency to clot too easily. Up to 900,000 Americans are affected by dangerous blood clots each year, and an estimated 60,000 to 100,000 die from them.
Understanding the specific triggers helps you recognize when you might be at higher risk and what’s actually happening inside your body when a clot forms.
The Three Conditions That Cause a Clot
Every blood clot traces back to some combination of three factors. First, the wall of a blood vessel gets damaged, whether from an injury, surgery, or the slow wear of conditions like high blood pressure. Second, blood flow slows down or pools in one area, which happens when you sit for hours without moving or when you’re on bed rest. Third, your blood itself becomes more prone to clotting than normal, a state that can be triggered by inflammation, hormones, cancer, or your genetics.
You don’t always need all three at once, but the more of these factors that overlap, the higher the risk. A person recovering from knee replacement surgery, for example, has vessel damage from the operation, limited mobility afterward, and an inflammatory response that thickens the blood. That triple hit explains why surgical patients receive blood-thinning treatment during recovery.
Venous Clots vs. Arterial Clots
Clots in veins and clots in arteries form through different mechanisms and cause different problems. Venous clots, the kind most people mean when they ask about blood clots, typically develop in the deep veins of the legs. Blood pools in these veins when you’re immobile, and if the blood is in a clot-prone state, a thrombus can grow. If that clot breaks loose and travels to the lungs, it becomes a pulmonary embolism, which can be fatal.
Arterial clots form differently. Over years, cholesterol-rich plaque builds up inside artery walls. That plaque can crack open or erode, and when it does, your body treats the exposed plaque like a wound and rapidly forms a clot over it. The clot blocks the artery, cutting off blood supply. When this happens in an artery feeding the heart, it causes a heart attack. In an artery feeding the brain, it causes a stroke.
Surgery and Hospitalization
Being hospitalized is one of the single most important risk factors for developing a blood clot. People admitted for acute medical illness face more than a tenfold increase in risk compared to those at home.
Surgery raises the stakes dramatically. In the first six weeks after an inpatient operation, the risk of a clot is roughly 70 times higher than normal, peaking around the third week after surgery. The type of surgery matters enormously. Hip or knee replacement carries the highest risk, at more than 220 times the baseline. Cancer surgery, fracture repair, and vascular procedures also carry very high risk, each roughly 90 times above normal. Even outpatient day surgeries raise the risk, though less steeply, at about 10 times normal. The elevated risk period lasts approximately 12 weeks after surgery, gradually tapering.
Sitting Still for Long Periods
Your calf muscles act as pumps that push blood back up toward your heart. When you sit or lie still for hours, those pumps stop working, and blood pools in your lower legs. This is why long-haul flights are a well-known trigger. Research tracking travelers found that the risk of a clot is about four times higher in the two weeks following a long flight, with the first week carrying the greatest danger. By the third week, the elevated risk is no longer significant.
Flights get the most attention, but any prolonged sitting carries the same mechanics: long car trips, desk work without breaks, or bed rest during illness. The common thread is stagnant blood flow in the legs.
Hormones, Pregnancy, and Birth Control
Estrogen increases the blood’s tendency to clot, which is why several hormone-related situations raise the risk. Combined oral contraceptives (those containing both estrogen and a progestin) increase clot risk two- to eightfold compared to not using them. For context, the baseline risk for women of childbearing age is low, about 6 in 10,000 per year. Certain newer formulations containing drospirenone push that number to roughly 10 in 10,000, a small absolute risk but a meaningful relative increase. Smoking, obesity, and a family history of clots compound that risk further.
Pregnancy raises clot risk fivefold compared to not being pregnant, with the weeks after delivery carrying even higher risk than the pregnancy itself. Postmenopausal hormone therapy increases risk two- to fourfold, with higher estrogen doses carrying greater risk.
Obesity and Chronic Inflammation
Carrying excess weight does more than slow you down physically. Fat tissue, especially visceral fat around the organs, actively changes blood chemistry in ways that promote clotting. People with a BMI over 30 face two to three times the normal risk of a blood clot, and those with a BMI above 40 face even higher odds.
The mechanism is a chain reaction. Fat tissue produces inflammatory signals that ramp up the activity of clotting factors in the blood and make platelets stickier. At the same time, visceral fat releases higher levels of a protein that blocks the body’s natural clot-dissolving system. So not only does the blood clot more readily, but the body also has a harder time breaking those clots down. On top of that, a hormone produced by fat cells directly enhances platelet clumping, adding yet another layer of risk.
Cancer and Blood Clots
Tumors have a well-documented ability to shift the blood toward a clot-prone state. About 10 percent of patients who show up with a blood clot will be diagnosed with a previously undetected cancer within two years. All active cancers raise clot risk, but certain types carry especially high risk: pancreatic cancer, lymphoma, brain tumors, liver cancer, leukemia, and cancers of the digestive tract. Cancer that has spread to other parts of the body raises the risk further, and chemotherapy adds another layer on top of that.
Inherited Clotting Disorders
Some people are genetically wired to clot more easily. The two most common inherited conditions involve mutations that affect how your blood regulates its clotting cascade. One, called Factor V Leiden, is carried by about 6 percent of white Americans, less than 1 percent of Black Americans, and roughly 2 percent of Hispanic Americans. It’s common in Europe and the Middle East but extremely rare in Asia and Africa. The second common mutation affects a clotting protein called prothrombin and follows a similar geographic pattern, present in about 2 to 3 percent of Europeans and very rare elsewhere.
Having one of these mutations doesn’t guarantee you’ll develop a clot. Many carriers go their entire lives without one. But when combined with other triggers, like surgery, birth control pills, or a long flight, the inherited tendency can tip the balance.
What a Blood Clot Feels Like
A deep vein clot in the leg typically causes swelling, pain or cramping (often starting in the calf), warmth in the affected area, and a change in skin color to red or purple. The swelling is usually the most noticeable feature and tends to affect only one leg. This differs from a muscle strain, which typically involves a clear moment of injury, localized tenderness, and bruising without significant swelling of the entire leg.
The tricky part is that deep vein clots can also occur without any noticeable symptoms at all. A pulmonary embolism, where the clot has traveled to the lungs, often announces itself with sudden shortness of breath, chest pain that worsens with deep breathing, a rapid heartbeat, or coughing up blood. These symptoms require emergency care.