Venous Thromboembolism
Venous Thromboembolism

Venous Thromboembolism Causes and Risk Factors


Causes of deep vein thrombosis

Venous thromboembolism (VTE) occurs in the veins that carry blood to your heart. A deep vein thrombosis (DVT) may occur if the flow of blood slows down in your body’s deep veins, if something damages the blood vessel lining, or if the makeup of the blood itself changes so that blood clots form more easily.

Causes of pulmonary embolism

A pulmonary embolism (PE) refers to an embolus from a deep vein blood clot that breaks loose and travels to the lungs, blocking an artery in the lung. Blood clots can develop in veins damaged by surgery or trauma, or they can develop as a result of inflammation in response to an infection or injury.

Why do blood clots form?

Blood clots form naturally at sites of injury to prevent bleeding. Damage to a vein causes certain factors in the blood to trigger the activity of the enzymes thrombin. Active thrombin then forms long protein strands that clump together with platelets and red blood cells to form clots.

What raises the risk for VTE?

Most often, there is more than one risk factor involved. The more risk factors you have, the greater the chance you have of developing VTE.


Knee and hip replacement surgery carry a high risk for VTE, as does peripheral and coronary artery bypass surgery, surgery to remove cancer, neurosurgery, abdominal surgery, and other major operations. Clotting is one of the body’s natural ways to heal the damage to your blood vessels from surgery. While you wear a cast or stay in bed to heal, your venous circulation slows because you stop moving as much as usual. This lack of movement increases the risk of blood clotting.

The risk of developing VTE is highest in the first 3 months after surgery and decreases with time. Ask your medical team about prevention strategies if you are scheduled for major surgery.

Other medical conditions

Certain medical conditions can increase your risk of developing a DVT. Some conditions are more closely linked to developing VTE than others and include the following:

  • Spinal cord injury, which can damage veins deep in your body and may cause paralysis, reducing blood flow
  • A broken hip or leg bone or other trauma
  • Cancers, such as advanced brain, breast, colon, or pancreatic cancer; and treatment for cancer, including chemotherapy and surgery
  • Heart conditions such as heart attack or congestive heart failure
  • Stroke
  • Obesity
  • Varicose veins, particularly if they are large or left untreated
  • Sickle cell disease, which makes the blood clot more easily
  • Infections, such as SARS-CoV-2, the virus that causes COVID-19

Learn more about how COVID-19 can lead to a blood clot in the lungs or deep veins, usually in the legs. Additionally, we offer information and resources on how we are working hard to support necessary COVID-19 research.

Not moving for long periods

Being still slows blood flow through the veins in your arms and legs, raising your risk of deep vein thrombosis. Especially when combined with other risk factors, not moving for long periods of time — such as when on bed rest in a nursing home or hospital setting, when you have a cast, or during a long flight — can increase your risk.

Slower blood flow can create a low-oxygen environment that makes it easier for clots to form or trap blood in places, such as the veins in the legs, where they have a tougher time breaking up.


VTE can occur at any age, but your risk increases as you age. Every 10 years after age 40, the risk of VTE almost doubles.

Family history

Your family history can affect your chances of developing VTE. Researchers have found dozens of genetic mutations (changes in the structures of your genes) that can increase your risk of VTE. Some changes make your blood more likely to clot. If your parents have these genetic changes, you may have inherited them.

Studies show that the risk of VTE among siblings of someone who has had VTE is more than double the risk among the general population.

Race or ethnicity

Studies suggest that how often people get VTE and how serious it is may vary by a person’s race and ethnicity.

In the United States, African Americans have the highest rates of VTE. Recent findings have revealed new genetic factors influencing a person’s risk for VTE, and those appear to be more common among African Americans.

VTE is less common among Asian Americans and Pacific Islanders.

Women and VTE

Women in their childbearing years are more likely to develop VTE than men of the same age.

  • Hormonal birth control and hormone therapy: Women who take birth control pills or get hormone therapy have an increased risk of clotting. Some medicines appear to put people at a higher risk than other medicines do, and the risk tends to be highest in the first few months after starting to take them. If you have other risk factors in addition to taking hormone-based medicines, keep in mind that the risk for blood clots may be even higher.
  • Pregnancy and giving birth: Women are at higher risk for VTE during the first 6 weeks after giving birth to a baby. The risk is also somewhat higher than usual during pregnancy. This may be due to hormones or other factors in the blood, changes in the way blood flows through your veins, or damage to your blood vessels during delivery. If you are required to be on bed rest while you are pregnant, the lack of movement can reduce blood flow through your veins. Surgery for a cesarean delivery may also increase your risk.

After menopause, women’s risk is lower than men’s.

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