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 the mother's genes have some sort of role, as the condition often runs in families.