Risk factors

Although anyone can develop blood clots and subsequent pulmonary embolism, certain factors can increase your risk.

Medical conditions and treatments

You're at higher risk if you or any of your family members have had venous blood clots or pulmonary embolism in the past.

In addition, some medical conditions and treatments put you at risk, such as:

• Heart disease. Cardiovascular disease, specifically heart failure, makes clot formation more likely.
• Cancer. Certain cancers — especially brain, ovary, pancreas, colon, stomach, lung and kidney cancers, and cancers that have spread — can increase the risk of blood clots, and chemotherapy further increases the risk. Women with a personal or family history of breast cancer who are taking tamoxifen or raloxifene also are at higher risk of blood clots.
• Surgery. Surgery is one of the leading causes of problem blood clots. For this reason, medication to prevent clots may be given before and after major surgery, such as joint replacement.
• Disorders that affect clotting. Some inherited disorders affect blood, making it more prone to clot. Other medical disorders such as kidney disease can also increase your risk of blood clots.
• Coronavirus disease 2019 (COVID-19). People who have severe symptoms of COVID-19 have an increased risk of pulmonary embolism.

Prolonged immobility

Blood clots are more likely to form during periods of inactivity, such as:

• Bed rest. Being confined to bed for an extended period after surgery, a heart attack, leg fracture, trauma or any serious illness makes you more vulnerable to blood clots. When the lower extremities are horizontal for long periods, the flow of venous blood slows and blood can pool in the legs, sometimes resulting in blood clots.
• Long trips. Sitting in a cramped position during lengthy plane or car trips slows blood flow in the legs, which contributes to the formation of clots.

Other risk factors

• Smoking. For reasons that aren't well understood, tobacco use predisposes some people to blood clot formation, especially when combined with other risk factors.
• Being overweight. Excess weight increases the risk of blood clots — particularly in people with other risk factors.
• Supplemental estrogen. The estrogen in birth control pills and in hormone replacement therapy can increase clotting factors in your blood, especially if you smoke or are overweight.
• Pregnancy. The weight of the baby pressing on veins in the pelvis can slow blood return from the legs. Clots are more likely to form when blood slows or pools.