Anything that prevents the blood from flowing or properly clotting can cause a blood clot. The main causes of deep vein thrombosis (DVT) are damage to a vein from surgery or inflammation and damage due to infection or injury.
Risk factors
Many things can increase the risk of developing deep vein thrombosis (DVT). The more risk factors you have, the greater your risk of DVT. Risk factors for DVT include:
Age. Being older than 60 increases the risk of DVT. But DVT can occur at any age.
Lack of movement. When the legs don’t move for a long time, the calf muscles don’t squeeze (contract). Muscle contractions help blood flow. Sitting for a long time, such as when driving or flying, increases the risk of DVT. So does long-term bed rest, which may result from a lengthy hospital stay or a medical condition such as paralysis.
Injury or surgery. Injury to the veins or surgery can increase the risk of blood clots.
Pregnancy. Pregnancy increases the pressure in the veins in the pelvis and legs. The risk of blood clots from pregnancy can continue for up to six weeks after a baby is born. People with an inherited clotting disorder are especially at risk.
Certain medications. Some medications increase the chances your blood will form a clot. These include birth control pills, hormone therapy drugs, glucocorticoids, and antidepressants.
Trauma. Having an injury that damages your veins, like a bone fracture, can cause a blood clot to develop.
Being overweight or obese. Being overweight increases the pressure in the veins in the pelvis and legs.
Smoking. Smoking affects how blood flows and clots, which can increase the risk of DVT.
Cancer. Some cancers increase substances in the blood that cause the blood to clot. Some types of cancer treatment also increase the risk of blood clots.
Heart failure. Heart failure increases the risk of DVT and pulmonary embolism. Because the heart and lungs don’t work well in people with heart failure, the symptoms caused by even a small pulmonary embolism are more noticeable.
Inflammatory bowel disease. Crohn’s disease or ulcerative colitis increase the risk of DVT.A personal or family history of DVT or PE. If you or someone in your family has had one or both of these conditions, you might be at greater risk of developing DVT.
Catheter. Having a catheter placed in a vein can increase the likelihood of a blood clot forming.
Genetics. Some people have DNA changes that cause the blood to clot more easily. One example is factor V Leiden. This inherited disorder changes one of the clotting factors in the blood. An inherited disorder on its own might not cause blood clots unless combined with other risk factors.
Sometimes, a blood clot in a vein can occur with no identifiable risk factor. This is called an unprovoked VTE.
In addition, several other health conditions can increase the risk for DVT. These include:
- cardiac problems like high blood pressure and heart failure
- sepsis
- Covid-19, tuberculosis, and other viral or bacterial illnesses
- asthma
- sleep apnea
- polycystic ovary syndrome (PCOS)
- diabetes