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Deep vein thrombosis (DVT)

Read about how and when deep vein thrombosis (DVT) can occur. In certain circumstances, such as being inactive for long periods, your risk of getting DVT increases.

Deep vein thrombosis (DVT) sometimes occurs for no apparent reason.

However, the risk of developing DVT is increased in certain circumstances.




In hospital

If you have to go into hospital for an operation or procedure, your risk of getting a blood clot increases. This is because DVT is more likely to occur when you're unwell or inactive, or less active than usual.

As a patient, your risk of developing DVT depends on the type of treatment you're having. You may be at increased risk of DVT if any of the following apply:

  • you're having an operation for an inflammatory or abdominal condition, such as appendicitis
  • you're confined to a bed, unable to walk, or spending a large part of the day in a bed or chair for at least three days

You may also be at increased risk of DVT if you're much less active than usual because of an operation or serious injury and have other DVT risk factors, such as a family history of the condition.

When you're admitted to hospital you'll be assessed for your risk of developing a blood clot and, if necessary, given preventative treatment.

Blood vessel damage




Medical and genetic conditions



During pregnancy, blood clots more easily. It's the body's way of preventing too much blood being lost during childbirth.



Having thrombophilia (a condition where the blood has an increased tendency to clot), or having a parent, or brother or sister, who's had a thrombosis, increase your risk of developing a DVT during pregnancy.

Other risk factors during pregnancy include:

  • being over 35 years old
  • being obese (with a BMI of 30 or more)
  • expecting two or more babies
  • having recently had a caesarean section
  • being immobile for long periods of time
  • smoking (find out how to stop smoking)
  • having severe varicose veins
  • dehydration

Low molecular weight heparin (LMWH) is usually used to treat pregnant women with DVT. LMWH is an anticoagulant, which means it prevents the blood clot getting bigger. It's given by injection and doesn't affect your developing baby.

Read more about DVT in pregnancy.

Contraceptive pill and HRT

The combined contraceptive pill and hormone replacement therapy (HRT) both contain the female hormone oestrogen. Oestrogen causes the blood to clot a bit more easily than normal, so your risk of getting DVT is slightly increased. There's no increased risk from the progestogen-only contraceptive pill.

Other causes


  • you're overweight or obese
  • you smoke
  • you're dehydrated 
  • you're over 60 – particularly if you have a condition that restricts your mobility


Hormone replacement therapy or HRT involves giving hormones to women when the menopause starts, to replace those that the body no longer produces.
Veins are blood vessels that carry blood from the rest of the body back to the heart.
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