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

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

Inactivity 

When you're inactive your blood tends to collect in the lower parts of your body, often in your lower legs. This is usually nothing to worry about because when you start to move, your blood flow increases and moves evenly around your body.

However, if you're unable to move for a long period of time – such as after an operation, because of an illness or injury, or during a long journey – your blood flow can slow down considerably. A slow blood flow increases the chances of a blood clot forming.

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 that takes longer than 90 minutes, or 60 minutes if the operation is on your leg, hip or abdomen
  • 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

If the wall of a blood vessel is damaged, it may become narrowed or blocked, which can cause a blood clot to form.

Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. Sometimes, blood vessel damage that occurs during surgery can cause a blood clot, particularly in operations on the lower half of your body.

Conditions such as vasculitis (inflammation of the blood vessels), varicose veins and some forms of medication, such as chemotherapy, can also damage blood vessels.

Medical and genetic conditions

Your risk of getting DVT is increased if you have a condition that causes your blood to clot more easily than normal. These conditions include:

Pregnancy

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

Venous thromboembolism (VTE) – DVT and pulmonary embolism – affects about one in 100,000 women of childbearing age.

DVTs are also rare in pregnancy, although pregnant women are up to 10 times more likely to develop thrombosis than non-pregnant women of the same age. A clot can form at any stage of pregnancy and up to six weeks after the birth.

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

Your risk of getting DVT is also increased if you or a close relative have previously had DVT and:

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

How blood clots

Blood contains cells called platelets and proteins known as clotting factors. When a blood vessel is cut, the platelets and clotting factors form a solid clot that acts as a plug to stop the wound bleeding.

Normally, blood clotting occurs when a blood vessel is damaged and bleeds. If the blood clots when a vessel isn't damaged, a clot can form within a vein or artery (thrombosis) and restrict the blood flow.

Page last reviewed: 27/04/2016

Next review due: 01/04/2019