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. Sometimes there are no noticeable symptoms. You can get DVT if you have certain medical conditions that affect how the blood clots. A blood clot in the legs can also develop if you don't move for a long time. For example, you might not move a lot when traveling a long distance or when you're on bed rest due to surgery, an illness or an accident. Deep vein thrombosis can be serious because blood clots in the veins can break loose. The clots can then travel through the bloodstream and get stuck in the lungs, blocking blood flow (pulmonary embolism). When DVT and pulmonary embolism occur together, it's called venous thromboembolism (VTE).