This describes an abnormal clot in a vein deep inside the calf or thigh. These are particularly dangerous as the clot can potentially break off, travel up the venous system to the heart and cause sudden onset of chest pain, shortness of breath and even death.  There is an increased incidence of deep vein thrombosis in people who sit still for a long time, such as a long plane ride, and also post-operatively. One baby aspirin per day is often effective in preventing deep vein thrombosis. Patients with a deep vein thrombosis may present with a painful, swollen leg that is tender to the touch with redness and hardness that can sometimes be felt through the skin and muscle. A diagnostic ultrasound or venography can be used to diagnose this potentially life-threatening condition.