Blood clots form when three conditions converge: damage to a blood vessel wall, sluggish or disrupted blood flow, and blood that clots more easily than normal. These three factors, first described in the 1800s and still central to modern medicine, explain virtually every blood clot, whether it forms in a leg vein after surgery or in a coronary artery during a heart attack. Up to 900,000 Americans develop a clot in their veins each year, and 60,000 to 100,000 die from them.
How Blood Clots Actually Form
Your blood vessels are lined with a smooth layer of cells that actively prevents clotting. This lining releases molecules that keep platelets from sticking and keeps the clotting cascade in check. When that lining is damaged, the protective signals disappear and the opposite happens: proteins flood the area that attract platelets, trigger clotting factors, and build a mesh of fibrin (the structural protein of a clot). This is the body’s normal repair system. Problems arise when it activates in the wrong place or at the wrong time.
Three overlapping conditions drive clot formation. First, vessel wall damage strips away the anti-clotting surface and exposes the raw tissue beneath, which is highly attractive to platelets. Second, abnormal blood flow, whether too slow or too turbulent, allows platelets and clotting proteins to pile up against the vessel wall instead of flowing smoothly past it. Third, changes in the blood itself can make it “stickier,” with higher levels of clotting proteins like fibrinogen or factor VIII tipping the balance toward coagulation. Most dangerous clots involve at least two of these three conditions working together.
Arterial Clots vs. Venous Clots
Clots in arteries and clots in veins form through different mechanisms and cause different problems. Arterial clots are the ones behind heart attacks and most strokes. They typically start with atherosclerosis, the slow buildup of cholesterol-laden plaques inside artery walls. Over years, immune cells cross into the artery wall, engulf cholesterol, and create a fatty core covered by a thin fibrous cap. When that cap thins to less than 65 micrometers (thinner than a human hair), it becomes vulnerable to rupture. If it tears open, the plaque’s core is exposed to flowing blood, triggering a rapid platelet pileup that can block the artery within minutes.
Venous clots, by contrast, usually form in the deep veins of the legs or pelvis. The primary driver is stagnant blood flow rather than plaque rupture. When blood pools in a vein for too long, clotting factors accumulate and platelets begin to adhere to the vessel wall. This is why prolonged immobility, whether from surgery, hospitalization, or a long flight, is such a consistent risk factor. A venous clot that breaks loose and travels to the lungs becomes a pulmonary embolism, which is fatal as the first symptom in roughly 25% of cases.
Surgery and Hospitalization
More than a third of venous clot cases diagnosed each year are tied to a recent hospital stay. Surgery is one of the strongest triggers because it combines all three clot-forming conditions at once: the procedure damages blood vessels, anesthesia and bed rest slow blood flow, and the body’s inflammatory response to tissue injury makes the blood more prone to clotting.
The risk is highest in the first week. A large meta-analysis found that 47% of post-surgical clots occurred during the first week after an operation, 74% by the end of the second week, and 90% by the third week. Major orthopedic procedures like hip and knee replacements carry especially high risk because they involve direct manipulation of large veins near the surgical site. This is why hospitals use compression devices and blood-thinning medications in the days and weeks after major surgery.
Hormonal Birth Control
Combined oral contraceptives (those containing both estrogen and a progestin) increase the risk of venous clots by roughly fourfold compared to not using hormonal birth control. To put that in perspective, the baseline rate is about 1.9 to 3.7 clots per 10,000 women per year, so even with the increase, the absolute risk remains low for most women.
The risk is not constant over time. It peaks sharply in the first three months of use, when the odds of a clot are about 12 times higher than for non-users. After a year of continuous use, the risk settles to roughly five times the baseline. The type of progestin in the pill also matters. Older formulations containing levonorgestrel carry a roughly 3.6-fold increase, while newer progestins like desogestrel and gestodene raise the risk by 5.6 to 7.3 times. This doesn’t mean older pills are “safe” and newer ones are “dangerous,” but it’s worth discussing with a provider if you have additional risk factors like smoking, obesity, or a family history of clotting.
Inherited Clotting Disorders
Some people are born with blood that clots too easily. The most common inherited clotting disorder is the Factor V Leiden mutation, carried by roughly 1% to 5% of people of European descent. In people who have already had a venous clot, the prevalence jumps to 10% to 20%, meaning it’s a significant contributor. This mutation makes one of the blood’s natural clotting proteins resistant to being shut off, so once clotting starts, it’s harder for the body to rein it in.
Other inherited conditions involve elevated levels of specific clotting factors or deficiencies in the proteins that normally dissolve clots. Having one of these mutations doesn’t guarantee you’ll develop a clot, but it lowers your threshold. A person with Factor V Leiden who also takes hormonal birth control, for example, faces a compounded risk far greater than either factor alone.
Cancer
An estimated one in five venous clots is related to cancer or its treatment. Tumors promote clotting through several routes. Cancer cells can directly produce proteins that activate the clotting cascade, including tissue factor (which kickstarts coagulation) and a unique enzyme called cancer procoagulant that triggers clotting through a pathway independent of the usual mechanisms. Tumors also release inflammatory signals that convert the normally anti-clotting surface of blood vessels into a pro-clotting one. On top of that, chemotherapy, surgery, and the immobility that comes with serious illness each add their own clotting risk.
In some cases, an unexplained blood clot is actually the first sign of an undiagnosed cancer. Cancers of the pancreas, lung, stomach, and brain carry particularly high clotting risk.
Inflammation and Autoimmune Disease
Chronic inflammatory conditions like lupus and rheumatoid arthritis increase clot risk even when the blood vessels themselves appear healthy. Inflammation and clotting are deeply intertwined: activated immune cells trigger the clotting cascade, and clotting proteins, in turn, amplify inflammation, creating a self-reinforcing cycle. Platelets and immune cells activate each other, unleash complement proteins (part of the immune system’s attack machinery), and shift blood chemistry toward coagulation.
This connection explains why people with chronic inflammatory diseases face a higher rate of heart attacks and strokes than the general population, even after accounting for traditional risk factors like cholesterol and blood pressure.
COVID-19 and Viral Infections
Severe COVID-19 brought the connection between infection and clotting into sharp public focus. The virus damages blood vessel linings throughout the body, triggering widespread activation of the complement system and clotting cascade simultaneously. In severe cases, this leads to tiny clots forming in small blood vessels across multiple organs, a condition called systemic microangiopathy, alongside larger clots in the lungs and legs. This pattern of widespread clotting, driven by an overactive immune response, can progress to multi-organ failure.
While COVID-19 was an extreme example, other serious infections can also shift the body toward a hypercoagulable state through similar mechanisms of endothelial damage and immune activation.
Immobility and Travel
Any period of prolonged stillness slows blood return from the legs and allows clotting factors to accumulate. The CDC notes that any travel lasting more than four hours, whether by plane, car, bus, or train, can increase clot risk. It’s not the altitude or cabin pressure on a plane that matters most; it’s sitting in the same position with your legs bent and compressed.
The same principle applies to desk work, long shifts in a seated position, or extended bed rest from illness. The risk compounds when combined with other factors. A person with Factor V Leiden on a 10-hour flight who is also dehydrated faces a meaningfully higher risk than someone without those added vulnerabilities. Moving your legs regularly, staying hydrated, and wearing compression stockings during long periods of sitting all help maintain blood flow.
Pregnancy
Women are five times more likely to develop a venous clot during pregnancy, childbirth, or the three months following delivery. This elevated risk exists because pregnancy naturally shifts blood chemistry toward easier clotting (an evolutionary adaptation to prevent hemorrhage during delivery), while the growing uterus compresses pelvic veins and slows blood return from the legs. The postpartum period carries the highest risk, as the combination of tissue damage from delivery, hormonal shifts, and reduced mobility converge.
The Common Thread
Nearly every blood clot cause traces back to the same three factors: vessel damage, sluggish or turbulent flow, and blood that’s been primed to clot. Most people who develop a clot have more than one risk factor stacking up at the same time. A hospitalized cancer patient on bed rest. A smoker with high blood pressure taking hormonal birth control. A post-surgical patient with an undiagnosed genetic mutation. Understanding which factors apply to you is the most practical step toward reducing your risk.