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Pre-eclampsia is thought to be caused by the placenta not developing properly due to a problem with the blood vessels supplying it. The exact cause isn't fully understood.

The placenta is the organ that links the mother's blood supply to her unborn baby's blood supply.

Food and oxygen pass through the placenta from mother to baby. Waste products can pass from the baby back into the mother.

To support the growing baby, the placenta needs a large and constant supply of blood from the mother.

In pre-eclampsia, the placenta doesn't get enough blood. This could be because the placenta didn't develop properly as it was forming during the first half of the pregnancy.

The problem with the placenta means the blood supply between mother and baby is disrupted.

Signals or substances from the damaged placenta affect the mother's blood vessels, causing high blood pressure (hypertension).

At the same time, problems in the kidneys may cause important proteins that should remain in the mother's blood to leak into her urine, resulting in protein in the urine (proteinuria).

In the initial stages of pregnancy, the fertilised egg implants itself into the wall of the womb (uterus). The womb is the organ a baby grows inside during pregnancy.

The fertilised egg produces root-like growths called villi, which help to anchor it to the lining of the womb.

The villi are fed nutrients through blood vessels in the womb and eventually grow into the placenta.

During the early stages of pregnancy, these blood vessels change shape and become wider.

If the blood vessels don't fully transform, it's likely that the placenta won't develop properly because it won't get enough nutrients. This may lead to pre-eclampsia.

It's still unclear why the blood vessels don't transform as they should. 

It's likely that inherited changes in your genes have some sort of role, as the condition often runs in families. But this only explains some cases.

Some factors have been identified that could increase your chances of developing pre-eclampsia.

These include:

• having an existing medical problem – such as diabetes, kidney disease, high blood pressure, lupus or antiphospholipid syndrome
• previously having pre-eclampsia

Some factors also increase your chances by a small amount.

If you have 2 or more of these together, your chances are higher:

• it's your first pregnancy – pre-eclampsia is more likely to happen during the first pregnancy than during any subsequent pregnancies
• it's been at least 10 years since your last pregnancy
• you have a family history of the condition – for example, your mother or sister has had pre-eclampsia
• you're over the age of 40
• you were obese at the start of your pregnancy – meaning you had a body mass index (BMI) of 35 or more
• you're expecting twins or more, such as twins or triplets

If you're considered to be at a high risk of developing pre-eclampsia, you may be advised to take a 75 to 150mg dose of aspirin every day from when you're 12 weeks pregnant until your baby is born.

Evidence suggests this can lower your chances of developing the condition.