Three factors may lead to formation of a clot inside a blood vessel.

 Damage to the inside of a blood vessel

 Changes in normal blood flow, including unusual turbulence or partial or complete blockage of blood flow

 Hypercoagulability - A rare state in which the blood is more likely than usual to clot Any component that contributes to 1 or more of these 3 criteria can cause DVT. The more common risk factors are as follows:

 Prolonged sitting, such as during a long plane or car ride

 Prolonged bed rest or immobility, such as after an injury or during an illness (stroke)

 Recent surgery, particularly orthopedic, gynecologic, or heart surgery

 Recent trauma to the lower body, such as fractures of the bones of the hip, thigh, or lower leg

 Obesity

 Heart attack or heart failure

 Recent childbirth

 Being at very high altitude, greater than 14,000 feet

 Use of estrogen replacement (hormone replacement therapy, or HRT) or birth control pills

 Malignant tumors (cancer)

 Rare inherited genetic changes in certain blood clotting factors

 Disseminated intravascular coagulation (DIC), a medical condition in which blood clotting occurs inappropriately, usually caused by overwhelming infection or organ failure If you have one DVT, you are more likely than the average person to have a second DVT