The cause of pre-eclampsia is not fully understood. However, it is thought that the placenta does not develop properly because of a problem with the blood vessels supplying it.
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 does not get enough blood. This could be because the placenta did not develop properly as it was forming during the first half of the pregnancy.
The problem with the placenta means that the blood supply between mother and baby is disrupted. Signals 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 valuable proteins that should remain in the mother's blood to leak into her urine, resulting in protein in the urine (proteinuria).
What causes problems with the placenta?
In the initial stages of pregnancy, the fertilised egg implants itself into the wall of the womb (uterus). The womb is a hollow 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 will eventually grow into the placenta. During the early stages of pregnancy, these blood vessels change shape and become wider.
If the blood vessels do not fully transform, it is likely that the placenta will not develop properly because it will not get enough nutrients. This may then lead to pre-eclampsia.
It is still unclear why the blood vessels do not transform as they should. It is likely that inherited changes in your genes have some sort of role, as the condition often runs in families.
Who is most at risk?
Some factors have been identified that could increase your chances of developing pre-eclampsia. These are listed below.
it is your first pregnancy - pre-eclampsia is more likely to happen during the first pregnancy than during any subsequent pregnancies
it has 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 previously had pre-eclampsia - there is an approximately 16% chance that you will develop the condition again in later pregnancies
you are over the age of 40
you have an existing medical problem - for example, diabetes, kidney disease or high blood pressure
you were obese
at the start of your pregnancy (you had a body mass index of 30 or more)
you are expecting multiple babies, such as twins or triplets
If you are considered to be at a high risk of developing pre-eclampsia, you may be advised to take a 75mg dose of aspirin (baby aspirin or low-dose aspirin) every day during your pregnancy from when you are 12 weeks pregnant until your baby is born. Evidence suggests this can help lower your chances of developing the condition.