What is deep vein thrombosis? Deep vein thrombosis (DVT) is a serious condition in which a blood clot called a thrombus develops in a vein located deep within the body. A deep vein thrombosis usually forms within a large vein in the thigh or calf area, or sometimes the pelvic area. DVT can also develop in the arm, but this is rare. Your veins are responsible for transporting oxygen-poor blood from the tissues of the body back to the heart. Blood in the veins is under lower pressure than blood in the arteries. Because of this, blood in the veins can pool and form blood clots more easily than in the arteries. Blood clots are most likely to form in the legs, where the effects of gravity can increase the pooling of blood. This can occur when you are sitting or lying in one position for an extended period of time, such as during recuperation from a disease or surgery. Certain medications and medical conditions can also cause a DVT by increasing the blood’s ability to clot or encouraging the clotting of blood in a vein. Normally, muscle movement in the legs helps move the blood through the veins and reduces the risk of developing blood clots. DVT can lead to a dangerous, potentially life-threatening complication known as a pulmonary embolism. This occurs when a blood clot breaks off and travels through the blood stream and lodges in the lungs, obstructing blood flow. DVT and pulmonary embolism are major health threats. The U.S. Centers for Disease Control and Prevention (CDC) estimates that between 350,000 and 600,000 Americans develop deep vein thrombosis and pulmonary embolisms each year (Source: CDC). DVT and pulmonary embolism are life-threatening emergency conditions. Seek immediate medical care (call 911) if you, or someone you are with, have symptoms of DVT or pulmonary embolism, such as unexplained leg pain or swelling, difficulty breathing, chest pain, or change in consciousness.