Find out more about DVT (deep vein thrombosis), a blood clot in a vein, usually in the leg. Deep vein thrombosis (DVT), is a blood clot that forms in a vein deep in the body, often in the leg. Learn about the causes, symptoms, and treatments. Deep vein thrombosis (DVT) occurs when a blood clot (thrombus) forms in one or more of the deep veins in the body, usually in the legs. Deep vein thrombosis can cause leg pain or swelling. A deep vein thrombosis (DVT) is a blood clot that forms in a vein deep inside your body. WebMD explains what causes it, as well as the symptoms and how you can prevent it. Deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, often in your leg. Symptoms include swelling, pain, redness and warmth in your leg. Deep vein thrombosis (DVT), which is a blood clot that develops in a deep vein, can cause pain, warmth, swelling, or redness (often in your calf muscle) that doesn’t go away. A DVT can be hard to. DVT is a blood clot that forms inside one of the deep veins of the body, most often in the calf or thigh. It can block normal blood return and, more dangerously, break loose and travel to the lungs as a pulmonary embolism.