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

Medical history

You're at higher risk if you or any of your family members have had venous blood clots or pulmonary embolism in the past. This may be due to inherited disorders that affect blood, making it more prone to clot.

In addition, certain medical conditions put you at risk, such as:

  • Heart disease. Cardiovascular disease, specifically heart failure, makes clot formation more likely.
  • Cancer. Certain cancers — especially pancreatic, ovarian and lung cancers, and many cancers with metastasis — can increase levels of substances that help blood clot, 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.

Prolonged immobility

Blood clots are more likely to form in your legs 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 far more vulnerable to blood clots. When the lower extremities are horizontal for long periods of time, the flow of venous blood slows and blood pools in the legs.
  • Long journeys. Sitting in a cramped position during lengthy plane or car trips slows blood flow, which contributes to the formation of clots in your legs.


Surgery is one of the leading causes of problem blood clots, especially seen after joint replacements of the hip and knee. During the preparation of the bones for the artificial joints, tissue debris may enter the bloodstream and contribute to causing a clot. Simply being immobile during any type of surgery can lead to the formation of clots. The risk increases with the length of time you're under general anesthesia. For this reason, most people undergoing a type of surgery predisposing them to DVT will receive medication before and after surgery to prevent clot formation.

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 women who smoke or have high blood pressure.
  • 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.
March 18, 2015