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.
Placenta
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 (see below).
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) and affecting her kidney function.
Waste products that should be removed from the mother’s blood and passed out in her urine remain in her blood. At the same time, valuable proteins that should remain in her blood are leaked into her urine, causing proteinuria (protein in the urine).
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, pear-shaped organ in which a baby grows during pregnancy. The 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 arteries change shape and become wider. If the arteries 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. Some research has suggested a possible link between pre-eclampsia, miscarriage and infertility. It is possible that the same medical reasons that cause infertility and miscarriage are responsible for the problems that lead to pre-eclampsia.
However, the exact nature of pre-eclampsia and why it affects certain people is still being researched.
Who is most at risk?
Some factors have been identified that could increase your chance 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 had pre-eclampsia in a previous pregnancy. There is an approximately 20% chance that you will develop the condition again in later pregnancies.
- You are a teenager or are over 40.
- You have an existing medical problem, for example, diabetes, kidney disease, migraines 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 (this places more strain on the placenta).
