When people think about blood clots, they often picture long flights or a major surgery—and those can absolutely raise risk. But clots can also develop in everyday situations, especially when several smaller risk factors stack up. Knowing what makes clots more likely can help you spot when it’s worth asking a clinician for personalized guidance.
How blood clots form (and why it matters)
Most dangerous clots involve either a clot in a deep vein (often in the leg) or a clot that travels to the lungs. Clotting is a normal, lifesaving process when you’re bleeding, but trouble starts when a clot forms inside a vessel without a good reason or doesn’t break down the way it should.
Clinicians often think about clot risk using a framework sometimes called “Virchow’s triad”: slowed blood flow, damage to the blood vessel lining, and increased “stickiness” of the blood (a higher tendency to clot). Many risk factors fit into one or more of those buckets, which is why combinations can matter more than any single issue.
Immobility and slowed blood flow (not just plane trips)
Long periods of sitting or lying down can slow circulation in the legs, making clots more likely. Travel is one example, but so are long car rides, being sick in bed, wearing a cast or brace, or having limited mobility after an injury.
Even everyday patterns—like working long hours seated with few breaks—can contribute, especially if other risks are present. Movement helps the calf muscles pump blood back toward the heart, so frequent standing and walking breaks are a simple protective habit for many people.
Surgery, hospitalization, and serious illness
Major surgery and hospitalization are well-known clot risks because they can combine immobility with inflammation and blood-vessel injury. Orthopedic procedures like hip or knee surgery are particularly associated with higher risk, which is why hospitals often use preventive strategies such as early walking, compression devices, or medication for selected patients.
Serious illnesses can also increase clot risk. Inflammation and changes in the body’s clotting balance can make blood more prone to clotting, and being less active during recovery adds to the effect.
Hormones: birth control, pregnancy, and hormone therapy
Estrogen-containing medications can increase the tendency for blood to clot. That includes certain birth control pills, patches, and rings, as well as some forms of menopausal hormone therapy. The level of risk varies based on the formulation and individual factors, so it’s worth discussing alternatives if you have other clot risks.
Pregnancy and the weeks after delivery also raise clot risk because the body naturally shifts toward clotting to reduce bleeding during childbirth. This is normal biology, but it means symptoms like new leg swelling or sudden shortness of breath shouldn’t be brushed off.
Cancer and cancer treatment
Cancer can increase clot risk for several reasons, including inflammation and substances released by tumors that affect clotting. Some treatments—such as certain chemotherapies, surgeries, and the presence of central venous catheters—can add additional risk.
Because the risk can be significant, oncology teams often watch closely for clot symptoms and may recommend preventive measures for some patients based on their overall risk profile.
Genetics and personal history
Some people inherit conditions that make clotting more likely (often called inherited thrombophilias). Having a personal history of a prior clot is also one of the strongest predictors of having another, especially if the first clot was unprovoked or occurred at a younger age.
Family history can be a clue, too—particularly if close relatives have had blood clots without a clear trigger. If clots “run in the family,” a clinician can help decide whether testing is useful and how that should influence medication or hormone choices.
Lifestyle and health conditions that quietly add risk
Several common health issues can nudge clot risk upward. Smoking is a major one, and risk can be higher when smoking is combined with estrogen-containing birth control. Obesity is also associated with increased clot risk, and it can overlap with reduced mobility and inflammation.
Age matters as well; clot risk generally rises as people get older. Chronic conditions such as heart failure or inflammatory diseases can also contribute, in part because they affect circulation and the body’s inflammatory state.
Dehydration, long sedentary stretches, and “stacking” small risks
People often focus on one big trigger, but clots can happen when several modest factors line up. Dehydration can contribute to thicker blood volume in some situations, and pairing that with hours of sitting—like a day of back-to-back meetings followed by a long drive—may not be ideal.
This doesn’t mean most sedentary days will cause a clot; they won’t. The key idea is that risk is cumulative, so paying attention to movement, hydration, and any additional medical risks can be protective over time.
If you’re wondering about your personal risk, it’s reasonable to bring it up at a routine visit—especially before starting estrogen-containing contraception or hormone therapy, planning surgery, or taking a long trip. And if you notice warning signs like new one-sided leg swelling or pain, chest pain, coughing blood, or sudden shortness of breath, seek urgent medical care since those symptoms can be serious and time-sensitive.