Deep vein thrombosis (DVT) occurs when a blood clot, or thrombus, forms in deep-lying veins, usually in the larger lower veins of the legs.
If the clot is large, it may interfere with circulation of the area and block blood flow from the legs back to the heart. A piece of the clot (embolus) may break off and be carried through the bloodstream.
The embolus can lodge in the lungs, blocking the blood flow and causing severe damage. A pulmonary embolus (a blood clot that travels to the lungs and blocks a lung vessel) can be fatal.