Anything that slows blood flow in deep veins can cause DVT. One of the most common risk factors is being immobile (not moving) for many hours. Factors that can increase your risk of having DVT are: having an injury such as a bone fracture being in hospital for surgery or a major illness sitting still for a long time, such as during a long-haul flight having conditions such as cancer or heart failure taking treatments such as the oral contraceptive pill (‘the pill') or hormone replacement therapy that contain the hormone oestrogen being pregnant having a genetic condition that makes your blood more likely to clot Lifestyle factors that can increase your risk include: being overweight smoking DVT can also happen suddenly, with no obvious cause.