Deep vein thrombosis (DVT) - Causes of deep vein thrombosis (DVT)

Deep vein thrombosis (DVT) sometimes occurs for no apparent reason. However, the risk of developing DVT is increased in certain circumstances.

Inactivity

When you are 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 are immobile (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

People in hospital have a higher risk of getting a blood clot because DVT is more likely to happen when you are unwell or inactive, or less active than you usually are.

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

you are having an operation that takes longer than 90 minutes, or 60 minutes if the operation is on your leg, hip or abdomen
you are having an operation for an inflammatory or abdominal condition, such as appendicitis
you are 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 a higher risk of DVT if you are much less active than usual because of an operation or serious injury and have other DVT risk factors, such as a family history.

When you are admitted to hospital you will 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 result in the formation of a blood clot.

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 vein wall), varicose veins and some forms of medication, such as chemotherapy, can also damage blood vessels.

Medical and genetic conditions

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

cancer – treatments such as chemotherapy and radiotherapy can increase this risk further
heart and lung disease
infectious diseases, such as hepatitis
inflammatory conditions, such as rheumatoid arthritis
thrombophilia – a genetic condition that makes your blood more likely to clot
Hughes syndrome – when your blood becomes abnormally "sticky"

Pregnancy

Pregnancy makes your blood clot more easily. This is your body's way of preventing too much blood loss during childbirth.

Around 1 in 1,000 pregnant women develop DVT at some point during their pregnancy. See DVT – Helen's story for an example of this.

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 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 or a close relative have previously had DVT and you are:

overweight or obese
a smoker
dehydrated
over 60 – particularly if you have a condition that restricts your mobility

Glossary

HRT: Hormone replacement therapy or HRT involves giving hormones to women when the menopause starts, to replace those that the body no longer produces.
Vein: Veins are blood vessels that carry blood from the rest of the body back to the heart.
Blood: Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Anaesthetic: Anaesthetic is a drug used to either numb a part of the body (local), or to put a patient to sleep (general) during surgery.
Chemotherapy: Chemotherapy is a treatment of an illness or disease with a chemical substance, e.g. in the treatment of cancer.
Inflammation: Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Heart: The heart is a muscular organ that pumps blood around the body.
Obesity: Obesity is when a person has an abnormally high amount of body fat.
Genetic: Genetic is a term that refers to genes- the characteristics inherited from a family member.