What Causes Deep Vein Thrombosis?

Blood clots can form in your body's deep veins when:

• Damage occurs to a vein's inner lining. This damage may result from injuries caused by physical, chemical, and biological factors. Such factors include surgery, serious injury, inflammation, or an immune response.
• Blood flow is sluggish or slow. Lack of motion can cause sluggish or slowed 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 usual. Certain inherited conditions (such as factor V Leiden) increase blood's tendency to clot. This also is true of treatment with hormone replacement therapy or birth control pills.