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If you have deep vein thrombosis (DVT for short), it means a blood clot has formed in one of your deep veins, probably one going through the muscles of one of your legs.

Veins are blood vessels that carry blood back to your heart from all over your body. If a vein in your leg gets partly or completely blocked by a blood clot, your leg can become swollen and painful.

You can get a clot below the knee (this is called calf DVT), or above the knee (this is called proximal DVT).

If part of the blood clot in your leg breaks off, it can go back through your heart and get stuck in an artery in one of your lungs. This is called a pulmonary embolism, and it can be dangerous. Pulmonary embolism is more likely to happen if the clot is above your knee than if it's below your knee. So a clot above the knee is more serious.^{1 2}

Normally, your blood forms clots to stop the bleeding if you injure yourself. But clots can also form inside your blood vessels. Some people are more likely to get clots in a vein in their leg than others. You're more likely to get DVT if you are:^{3 4 5 6}

• Older
• Overweight.

You are also more likely to get DVT if you:^{3 4 5 6}

• Have cancer
• Have had DVT before
• Have recently had a fracture
• Have varicose veins
• Have inflammatory bowel disease (conditions which cause inflammation of your bowel, such as Crohn's disease and ulcerative colitis)
• Take steroid pills regularly
• Sit or lie for long periods without moving much
• Have an operation, especially on your leg
• Have a problem with blood clotting (these problems can run in families).

If you are a woman, your chance of getting DVT is higher if you are:

• Pregnant
• Taking birth control pills or hormone replacement therapy (HRT).

Some research has found that long-distance travel increases your chance of having DVT. For example, one study suggested that, during long-distance flights, about 1 in 10 passengers older than 50 are at risk of getting DVT without any symptoms.⁷ A more recent study suggested that 1 in 100 people traveling long distances by air may get symptoms of a blood clot.⁸ To learn more about the symptoms, see What are the symptoms of deep vein thrombosis?

The higher risk could be because, during long-distance journeys, people are sitting still and not doing any exercise for long periods. But doctors aren't quite sure. It may be that people who get DVT while traveling long distances are at risk for other reasons.

If you have a long journey to make, you can do things that might lower your risk of getting DVT. To learn more, see Long-distance travel and DVT.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.