Deep vein thrombosis (DVT) sometimes occurs in normal veins for no apparent reason. However, the risk of developing DVT is increased in certain circumstances.
Immobility (lack of movement)
If you are immobile, the blood flow in your veins slows down and increases the chances of a blood clot occurring. DVT often develops after surgical procedures that last for more than 30 minutes. Under anaesthetic, your legs are still, and the blood flow in your veins slows down.
If you are due to have surgery, you may be given a daily injection of heparin (see the 'treatment' and 'prevention' sections) to reduce your chances of getting a DVT. Illnesses and injuries that cause immobility, such as a stroke, increase the risk of DVT. Also, long journeys by plane, train, and car may increase the risk slightly.
Previous DVT and vein damage
If the inside lining of your vein becomes damaged, your chances of getting DVT are increased. Conditions such as vasculitis (inflammation of the vein wall) and some forms of medication, such as chemotherapy, can damage veins and increase the risk of DVT. Also, DVT can damage the lining of your vein, increasing your chances of having another DVT in the future.
Medical and genetic conditions
People who have cancer, or heart failure, have an increased risk of getting DVT. Your risk of DVT is increased if you have a condition that causes your blood to clot more easily than normal. There are a number of medical conditions which can cause this to happen. Also, some rare genetic conditions, such as Factor V Leiden thrombophilia, which causes the blood to clot abnormally, can increase your chances of getting DVT.
Pregnancy
Pregnancy increases your chances of getting DVT, with approximately 1 in 1,000 pregnant women developing the condition.
Contraceptive pill and hormone replacement therapy (HRT)
The combined contraceptive pill, and hormone replacement therapy (HRT) both contain the female hormone, oestrogen. Oestrogen causes the blood to clot slightly more easily, so if you are taking either a combined contraceptive pill, or HRT, your risk of getting DVT is slightly increased. There is no increased risk from the progestogen-only contraceptive pill.
Other causes
Your risk of developing DVT is also increased if you are:
- obese,
- a smoker, or
- over 40 years of age (particularly if you have a condition that restricts your mobility).