Learn About Blood Clot Prevention and Risk Factors

Understanding Blood Clots and Why They Form

A blood clot is a clump of blood that has thickened and solidified. Your body forms clots naturally to stop bleeding when you get a cut or injury. Without this ability, even small wounds would bleed continuously. However, blood clots can also form inside blood vessels without an obvious injury, and these clots can become dangerous if they block blood flow to vital organs.

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Blood clots develop through a complex biological process. When a blood vessel is damaged, your body triggers a series of events involving proteins called clotting factors. These factors work together to create a mesh-like structure that traps blood cells, forming a solid plug. This process normally takes just a few minutes and is essential for survival. The problem occurs when clots form in the wrong place or grow too large.

There are two main types of dangerous blood clots. Deep vein thrombosis (DVT) occurs when a clot forms in deep veins, usually in the legs. Pulmonary embolism (PE) happens when a clot travels through the bloodstream and lodges in the lungs, blocking blood flow. PE is particularly serious because it can prevent oxygen from reaching the lungs. According to the Centers for Disease Control and Prevention, up to 900,000 Americans are affected by blood clots each year, with approximately 60,000 to 100,000 dying from PE.

Understanding how clots form helps explain why certain conditions and activities increase your risk. Your blood flows smoothly through healthy vessels because the vessel walls are smooth and your blood contains natural chemicals that keep it liquid. When vessel walls become damaged, blood flow slows, or the blood's composition changes, clots become more likely to form. This is why people with certain medical conditions face higher risks.

Practical Takeaway: Blood clots are a normal part of wound healing, but abnormal clots inside blood vessels can be life-threatening. Knowing what increases your clot risk helps you take steps to prevent problems before they start.

Major Risk Factors for Blood Clot Development

Several well-established risk factors increase your chances of developing dangerous blood clots. These fall into three general categories: factors related to blood vessel injury, blood flow problems, and changes in blood composition. Understanding these categories helps explain why certain groups of people face higher risks.

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Immobility is one of the most significant preventable risk factors. When you stay still for extended periods, blood pools in your leg veins instead of flowing freely back to your heart. This happens during long airplane flights, car trips, bed rest, or even sitting at a desk for many hours. Research shows that people who are immobilized for more than four hours face increased clot risk. This is why you hear recommendations to get up and move around during long flights or after surgery.

Surgery itself substantially increases clot risk for several reasons. The surgical procedure damages blood vessels, anesthesia affects how blood flows, and post-surgery immobility slows circulation. Hip and knee replacement surgeries carry particularly high risks because they involve large joints and typically require recovery periods with limited movement. Cancer surgery patients also face elevated risks. Studies indicate that surgery-related clots occur in 40 to 60 percent of orthopedic surgery patients who don't receive preventive treatment.

Cancer and its treatments significantly increase clot risk. Cancer patients are four to seven times more likely to develop blood clots than the general population. This happens because cancer cells release substances that promote clotting, cancer treatments can damage blood vessels, and cancer patients often have reduced mobility. Certain cancer types, including lung, pancreatic, and ovarian cancers, carry higher clot risks than others.

Pregnancy and the postpartum period create a natural clotting tendency. Pregnancy hormones change blood composition and slow circulation, especially in leg veins. The risk increases further during the last trimester and continues for six weeks after delivery. Cesarean delivery increases risk compared to vaginal delivery, and the risk is higher for women carrying multiples or those over age 35.

Heart disease, including heart failure and recent heart attacks, increases clot formation because the heart pumps less effectively, slowing blood circulation. Atrial fibrillation, an irregular heart rhythm, creates conditions where blood pools and clots form more easily. Approximately 15 to 20 percent of stroke patients have atrial fibrillation-related clots.

Practical Takeaway: The most controllable risk factors involve movement and activity level. Even if you have medical conditions that increase risk, staying as active as your doctor recommends can reduce your clot chances.

Medical Conditions and Medications That Increase Risk

Certain chronic health conditions create long-term increased clot risk. Understanding which conditions affect you helps guide conversations with your healthcare provider about prevention strategies.

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Obesity substantially increases clot risk through multiple mechanisms. Excess body weight increases blood pressure, which stresses blood vessels. Obesity also changes blood chemistry, making it more likely to clot. Studies show that people with a body mass index over 30 face significantly higher clot risks, and the risk increases further with greater weight. The risk remains elevated even after weight loss, though it does decrease.

Diabetes increases clot risk by damaging blood vessel walls and changing blood composition. High blood sugar levels promote inflammation and make blood more likely to clot. People with diabetes who also have other risk factors face compounded danger.

High blood pressure damages blood vessel walls, creating conditions where clots form more easily. This is one reason why controlling blood pressure through medication, diet, and exercise matters for overall health.

Kidney disease impairs the balance of clotting factors in your blood, increasing clot risk. Dialysis patients face particularly high risks.

Many medications increase clotting risk, most notably hormone-based treatments. Combined oral contraceptives increase clot risk by three to four times compared to non-users, with higher risks for formulations containing specific types of progestin. Hormone replacement therapy used for menopausal symptoms carries similar risks. These medications change blood clotting factors and blood vessel function.

Tamoxifen, used to treat breast cancer, increases clot risk significantly. Blood thinning medications like warfarin and direct oral anticoagulants are actually prescribed to prevent clots in high-risk patients, but getting the dosage right requires careful monitoring.

Certain supplements and herbs can increase clotting, though this varies by individual. These include vitamin K supplements and some herbal products. Always inform your healthcare provider about supplements you take.

Smoking damages blood vessel walls and increases clotting tendency. The risk is dose-dependent, meaning more smoking causes greater risk, but even light smoking increases risk compared to never smoking.

Practical Takeaway: If you take medications or have chronic conditions listed here, discuss clot prevention strategies with your healthcare provider rather than stopping medications on your own. Your provider can recommend appropriate preventive measures based on your individual situation.

Genetic and Family History Risk Factors

Some people inherit conditions that make their blood more prone to clotting. These genetic factors account for roughly 30 to 40 percent of blood clot cases. Understanding whether you have a family history of clots can help guide preventive discussions with your healthcare provider.

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Factor V Leiden is the most common inherited clotting disorder in people of European descent, affecting approximately 3 to 7 percent of this population. People with this condition have a genetic variation that makes their blood clot more easily. Those who inherit the variation from one parent have five times higher clot risk, while inheriting it from both parents increases risk by 50 to 100 times. However, many people with Factor V Leiden never develop clots, suggesting that other factors must be present to trigger clot formation.

Prothrombin gene mutation is the second most common inherited clotting disorder, affecting 1 to 3 percent of people of European descent. This genetic variation increases clotting factor levels in the blood, making clot formation more likely. Like Factor V Leiden, carrying this variation doesn't guarantee clot development but increases risk, especially when combined with other factors.

Antithrombin, protein C, and protein S deficiencies are rarer inherited conditions that affect natural blood-thinning systems. People with these conditions lack adequate amounts of proteins that normally prevent excessive clotting. These deficiencies carry higher clot risks than the more common genetic variations.