Blood clots form when your blood thickens and clumps into a semi-solid mass, usually in response to an injury, slow blood flow, or a change in your blood’s chemistry. 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. Understanding what triggers clotting helps you recognize whether you’re at higher risk and what the warning signs look like.
Three Conditions That Trigger a Clot
Nearly every blood clot traces back to one or more of three overlapping problems: slow blood flow, damage to a blood vessel wall, or blood that clots too easily. These three factors, first described over 150 years ago, still form the foundation of how doctors think about clot risk today.
Slow or stagnant blood flow. When blood pools instead of moving steadily through your veins, clotting proteins have more time to stick together. This is why sitting for long stretches, bed rest after surgery, or even a tumor pressing on a vein can set the stage for a clot.
Damage to a blood vessel. Surgery, a catheter, a broken bone, or even inflammation from an infection can injure the inner lining of a vein or artery. Your body responds by activating its clotting system to seal the wound, but sometimes that response overshoots and creates a dangerous clot.
Blood that clots too easily. Certain conditions make your blood more “sticky” than normal. Cancer, inherited clotting disorders, pregnancy, dehydration, and some medications all shift the balance toward clotting. Most people who develop a clot have at least two of these three factors working together.
Venous Clots vs. Arterial Clots
Not all blood clots are the same. Venous clots form in your veins, where blood moves slowly, and they tend to be rich in a protein called fibrin along with trapped red blood cells. These are the classic deep vein thrombosis (DVT) clots that form in your legs or pelvis. They develop on a largely intact blood vessel wall, driven more by stagnant flow and clotting imbalances than by physical damage.
Arterial clots are a different animal. They form in arteries, where blood moves fast and under high pressure, usually after a fatty plaque in the artery wall ruptures. These clots are packed with platelets rather than fibrin, giving them a lighter color. Arterial clots are the ones behind most heart attacks and strokes. The distinction matters because the two types are treated with entirely different medications.
Major Risk Factors for Blood Clots
Surgery and Hospitalization
Major surgery is one of the strongest triggers for blood clots, especially operations involving the abdomen, pelvis, hips, or legs. You’re lying still for hours during the procedure, your blood vessels may be directly injured, and the body’s inflammatory response ramps up clotting. Blood clots are the fifth most common reason for unplanned hospital readmissions after surgery overall, and the third most common after hip or knee replacement specifically. This is why hospitals routinely use compression devices and blood-thinning medications around these procedures.
Prolonged Sitting and Travel
Any travel lasting more than four hours, whether by plane, car, bus, or train, raises your risk. The combination of cramped seating, dehydration from low cabin humidity or skipping water, and hours without moving your legs creates ideal conditions for a clot to start forming in the deep veins of your calves. The risk isn’t unique to flying. Long car trips carry the same concern. Getting up to walk, flexing your ankles while seated, and staying hydrated all help keep blood moving.
Hormonal Birth Control
Estrogen-containing birth control pills roughly triple the risk of a venous clot compared to not using hormonal contraception. The overall relative risk sits around 3.5 times higher for users, with some variation depending on the specific type of progestin in the pill. Newer formulations containing desogestrel carry slightly higher risk (about 3.8 times) compared to older levonorgestrel-based pills (about 2.8 times). In absolute terms the risk is still low for most young, healthy women, but it climbs significantly if you also smoke, are obese, or have an inherited clotting disorder.
Cancer
Cancer is one of the most potent drivers of blood clots. Tumors actively manipulate the clotting system in several ways: they release tiny particles that trigger clotting, increase platelet production, and promote inflammation that makes blood vessel walls stickier. The risk varies by cancer type. Pancreatic tumors release high levels of a clotting activator directly into the bloodstream. Ovarian cancer stimulates the liver to produce more platelets. Lung cancer drives up white blood cell counts that release web-like structures trapping blood cells into clots. Chemotherapy drugs add further risk on top of the cancer itself.
Inherited Clotting Disorders
Some people are genetically wired to clot more easily. The most common inherited condition, Factor V Leiden, affects roughly 5% of people of European descent. If you’ve inherited it from one parent, you have about a 5% chance of developing an abnormal blood clot by age 65. That might sound modest, but it stacks on top of every other risk factor. A person with Factor V Leiden who also takes estrogen-containing birth control, for example, faces a much steeper combined risk than either factor alone would suggest.
Dehydration
When you’re dehydrated, the water content of your blood drops. This makes blood thicker and slower-moving, giving clotting proteins more opportunity to aggregate. Dehydration alone rarely causes a clot in an otherwise healthy person, but it amplifies other risks. It’s one reason clot risk rises during long flights (dry cabin air plus limited water intake) and during illness with vomiting or diarrhea.
Other Common Risk Factors
- Pregnancy: The growing uterus compresses pelvic veins, slowing blood return from the legs, while hormonal changes make blood clot more readily.
- Obesity: Excess weight increases pressure on veins in the pelvis and legs and promotes chronic low-grade inflammation.
- Immobility from injury: A cast, splint, or any reason you can’t move a limb normally reduces the muscle contractions that normally push blood through your veins.
- Smoking: Damages blood vessel walls and makes platelets stickier.
What a Blood Clot Feels Like
A deep vein clot in the leg typically causes pain or tenderness (often described as a cramp that doesn’t go away), swelling in one leg, warmth over the affected area, and skin that looks red or discolored. These symptoms usually appear in just one leg, which helps distinguish a clot from general muscle soreness.
The more dangerous scenario is when a clot breaks free and travels to the lungs, becoming a pulmonary embolism. Symptoms include sudden, unexplained shortness of breath, sharp chest pain that worsens with breathing, coughing (sometimes with blood), and fainting. About 25% of people who have a pulmonary embolism experience sudden death as their first symptom, with no warning signs beforehand. That statistic underscores why prevention and early recognition of leg clots matter so much.
Why Multiple Risk Factors Compound
Blood clots rarely result from a single cause. They almost always emerge from a combination: a long flight plus dehydration, surgery plus a genetic clotting tendency, cancer plus chemotherapy plus bed rest. Each individual factor nudges your clotting system a little closer to tipping over, and the combination pushes it past the threshold. This is why your risk profile changes over time. A 25-year-old on birth control who takes a long-haul flight while dehydrated faces a meaningfully different risk than the same person sitting at their desk at home, well-hydrated, and off hormonal contraception.
Knowing your personal risk factors lets you take targeted precautions: staying mobile during travel, discussing clotting risk before starting hormonal birth control, and being vigilant about leg symptoms after surgery or during cancer treatment.