Thrombosis occurs when blood clots form in deep veins in the body. They most often occur in the legs and cause pain and swelling in the affected areas. Blood clots may develop from a lack of blood circulation, from sitting for long periods of time, or due to a genetic disorder.When a blood clot forms in the veins, it is known as venous thromboembolism. This can cause deep vein thrombosis and pulmonary embolisms.Thrombosis is the formation of a blood clot, known as a thrombus, within a blood vessel. It prevents blood from flowing normally through the circulatory system.