Deep venous thrombosis or DVT is a condition where a blood clot forms in one of the major deep veins, usually of the lower leg, thigh or pelvis. It can cause pain and swelling of the affected leg and its most dangerous consequence is the development of pulmonary embolism (blood clot in the lung). Rarely, DVT may occur in a deep vein in the arm.
There are two types of veins in the leg – superficial and deep. The deep veins lie under the muscles while the superficial veins lie just under the skin. Blood clots may sometimes form in the superficial veins but cause milder problems than when the deep veins are involved.