Deep vein thrombosis (DVT) occurs when a blood clot forms in a vein deep in your body, usually in the legs. Such a clot is dangerous because the clot may break loose, travel through your bloodstream, and block arteries in your lungs, causing permanent damage or death.

DVT may occur when the blood moves through deep veins in your legs more slowly than normal or when there is some factor that makes your blood more likely to clot. When you are bedridden (after surgery, for example) or when you sit still for a long time (such as during a long plane flight), your blood moves more slowly. Blood pools in the larger veins of your legs, and clots may form. Also, injury, major illness, and some medicines increase the tendency for blood to clot.

Your risk of having DVT increases if you have some conditions, including immobility, surgery, a hip or leg fracture, pelvic surgery, stroke, congestive heart failure, cancer, and varicose veins.

Smoking cigarettes also increases the risk you will have a blood clot.