Deep Vein Thrombosis (DVT) occurs when blood moves too slowly through your veins, and a blood clot is formed. Typically, in a healthy vein, the pressure from vein walls creates a compression to help push the blood through the vein. In a damaged vein, venous disease causes the blood to pool and expands the veins, causing blood to flow slower. DVT typically occurs in the lower leg, thigh, or pelvis.