1. Injury to a vein (fracture, severe muscle injury, major surgery) 2. Slow blood circulation (confinement to bed, limited movement, paralysis, sitting for extended periods, especially with crossed legs) 3. Increased estrogen (birth control pills, hormone replacement therapy, pregnancy, post-partum up to 3 months) 4. Chronic medical conditions (heart disease, lung disease, cancer and its treatment, inflammatory bowel disease) 5. Other factors (previous DVT or PE, family history of DVT or PE, increased age, obesity, inherited clotting disorders)