If blood moves too slowly through your veins, it can cause a clump of blood cells called a clot. When a blood clot forms in a vein deep inside your body, it causes what doctors call “deep vein thrombosis” (DVT). This is most likely to happen in your lower leg, thigh, or pelvis. But it can occur in other parts of your body, too.
Deep vein thrombosis can develop if you have certain medical conditions that affect how your blood clots. It can also happen if you don't move for a long time, such as after surgery or an accident, or when you're confined to bed.
Deep vein thrombosis can be very serious because blood clots in your veins can break loose, travel through your bloodstream and lodge in your lungs, blocking blood flow.