Blood clots may form for various reasons. Severe trauma, such as from an auto accident, may cause internal blood clotting. Blood clots may also be created during surgery. For example, some side effects of open-heart surgery may be caused by tiny blood clots that form during use of the heart-lung machine (microembolization).

Blood clots may also be formed by or in association with: 

• Atherosclerosis
  
  
• Heart attack or stroke
  
  
• Valvular heart disease
  
  
• Heart failure
  
  
• Infection
  
  
• Cancer
  
  
• Autoimmune disorders (e.g., lupus or rheumatoid arthritis)
  
  
• Bleeding disorders (e.g., hemophilia)
  
  
• Inflammatory bowel disease (e.g., ulcerative colitis and Crohn's disease)
  
  
• History of varicose veins or other vascular conditions
  
  
• Pregnancy
  
  
• High blood pressure (hypertension)
  
  
• Thrombocythemia (in which greater numbers of platelets are produced)
  
  
• Atrial fibrillation

In addition, there are a number of risk factors that can increase the chances of developing a blood clot. These risk factors include:

• Smoking
  
  
• Elevated levels of homocysteine (homocysteinemia), which may be treated with diet and folic acid supplements
  
  
• Obesity (body mass index greater than 30)
  
  
• Prolonged lack of exercise
  
  
• Use of contraceptive pill or patch 
  
  
• Advanced age
  
  
• Genetic factors (e.g., inheriting a tendency to develop deep vein thrombosis)
  
  
• Sitting or laying in one position for a prolonged period of time
  
  
• Having a mechanical heart valve
  
  
• Hypercoagulable state, where genetic (e.g., Factor V Leiden) or acquired (e.g., trauma, pregnancy, Heart failure) factors affect blood clotting due to abnormal levels of blood-clotting proteins