Definition of Deep venous thrombosis:

Deep venous thrombosis is the formation of a blood clot in a vein that is deep inside a part of the body, usually the legs.

Causes, incidence, and risk factors:

Deep venous thrombosis (DVT) mainly affects the large veins in the lower leg and thigh. The clot can block blood flow and cause swelling and pain. When a clot breaks off and moves through the bloodstream, this is called an embolism. An embolism can get stuck in the brain, lungs, heart, or other area, leading to severe damage.

Blood clots may form when something slows or changes the flow of blood in the veins. Risk factors include:

• After a pacemaker catheter has been passed through the vein in the groin
• Bedrest
• Cigarette smoking
• Family history of blood clots
• Fractures in the pelvis or legs
• Giving birth within the last 6 months
• Heart failure
• Obesity
• Recent surgery (especially hip, knee, or female pelvic surgery)
• Too many blood cells being made by the bone marrow (polycythemia vera), causing the blood to be thicker and slower than normal

You're also more likely to develop DVT if you have any of the following conditions:

• Blood that is more likely to clot (hypercoagulability)
• Cancer
• Taking estrogens or birth control pills. This risk is even higher if you smoke.

DVTs are most common in adults over age 60, but can occur at any age.

Sitting for long periods when traveling can increase the risk of DVTs. This is most likely when one or more of the risk factors listed above are also present.