A blood clot embedded in one of the major deep veins of the legs, arms or pelvis. The clot blocks blood flow through these veins preventing blood from being brought back to the heart. Smoking, certain inherited (genetic) conditions (including factor V Leiden mutation, protein C deficiency, protein S deficiency, and antithrombin III deficiency), birth control pills, prolonged sitting, cancer, bed rest, giving birth, intravenous catheters, and recent fractures increase the risk of DVT. If the blood clot breaks away it can travel through the heart and lodge in the pulmonary artery causing a life threatening disease called pulmonary embolus.