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Deep vein thrombosis (DVT) can be dangerous. So it's very important to go to your doctor if you have any symptoms of DVT.

The most obvious symptoms of deep vein thrombosis (DVT) are swelling or pain in the calf or thigh of one leg, or both swelling and pain. To learn more, see What are the symptoms of deep vein thrombosis?

In about 1 in 2 people who have a clot that starts in their calf, the clot spreads to above their knee.¹ Blood clots that are above the knee are more likely to lead to a pulmonary embolism, which is dangerous. Pulmonary embolism happens when a blood clot travels to one of your lungs and gets stuck in an artery that brings blood to this lung.

So if your doctor thinks you have a clot in your calf, it's important to have a check again in seven days to see whether the clot has traveled above your knee.

Without treatment, in about 1 in 10 people with a clot above their knee, the clot will lodge in the artery of a lung (a pulmonary embolism). And as many as 1 in 10 people who get a pulmonary embolism die.²

Each year, about 1 in 4 people who have a clot below the knee get another one.^{3 4} DVT is more likely to come back if you have a blood-clotting problem or cancer. If you have cancer, your blood may tend to clot more. This can be caused either by the disease or by the treatment for it.

You are less likely to get DVT again if it happened after you had an operation.⁵

Following DVT, you can get swelling, open sores (ulcers) and permanent damage to tissue around the blocked vein. Doctors call this post-thrombotic syndrome. You are more likely to get this if the clot is above your knee.

In some people, DVT may clear up on its own, without treatment. Some people may never even know they had DVT. Still, if you are worried that you might have DVT, you should see your doctor at once.

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.