What causes Deep Vein Thrombosis ?
Blood clots can form in your body's deep veins if:
                
A vein's inner lining is damaged. Injuries caused by physical, chemical, or biological factors can damage the veins. Such factors include surgery, serious injuries, inflammation, and immune responses.
                
Blood flow is sluggish or slow. Lack of motion can cause sluggish or slow blood flow. This may occur after surgery, if you're ill and in bed for a long time, or if you're traveling for a long time.
                
Your blood is thicker or more likely to clot than normal. Some inherited conditions (such as factor V Leiden) increase the risk of blood clotting. Hormone therapy or birth control pills also can increase the risk of clotting.