Although they're rare, a number of complications can develop if pre-eclampsia isn't diagnosed and monitored.
These problems can affect both the mother and baby.
Eclampsia describes a type of convulsion or fit (involuntary contraction of the muscles) that pregnant women can experience, usually from 20 weeks of pregnancy or after the birth, usually within 48 hours.
Eclampsia is quite rare in the UK.
During an eclamptic fit, the mother's arms, legs, neck or jaw will twitch involuntarily in repetitive, jerky movements.
She may lose consciousness and may wet herself. The fits usually last less than a minute.
While most women make a full recovery after having eclampsia, there's a small risk of permanent disability or brain damage if the fits are severe.
A medicine called magnesium sulfate can halve the risk of eclampsia and reduce the risk of the mother dying.
It's now widely used to treat eclampsia after it's occurred and treat women who may be at risk of developing it.
HELLP syndrome is a rare liver and blood clotting disorder that can affect pregnant women.
It's most likely to occur immediately after the baby is delivered, but can appear any time after 20 weeks of pregnancy, and in rare cases before 20 weeks.
The letters in the name HELLP stand for each part of the condition:
HELLP syndrome is potentially as dangerous as eclampsia, and is slightly more common.
The only way to treat the condition is to deliver the baby as soon as possible.
Once the mother is in hospital and receiving treatment, it's possible for her to make a full recovery.
The blood supply to the brain can be disturbed as a result of high blood pressure. This is known as a cerebral haemorrhage, or stroke.
If the brain doesn't get enough oxygen and nutrients from the blood, brain cells will start to die, causing brain damage and possibly death.
The mother's blood clotting system can break down. This is known medically as disseminated intravascular coagulation.
This can either result in too much bleeding because there aren't enough proteins in the blood to make it clot, or blood clots developing throughout the body because the proteins that control blood clotting become abnormally active.
These blood clots can reduce or block blood flow through the blood vessels and possibly damage the organs.
Babies of some women with pre-eclampsia may grow more slowly in the womb than normal.
This is because the condition reduces the amount of nutrients and oxygen passed from the mother to her baby.
These babies are often smaller than usual, particularly if the pre-eclampsia occurs before 37 weeks.
If pre-eclampsia is severe, a baby may need to be delivered before they're fully developed.
This can lead to serious complications, such as breathing difficulties caused by the lungs not being fully developed (neonatal respiratory distress syndrome).
In these cases, a baby usually needs to stay in a neonatal intensive care unit so they can be monitored and treated.
Some babies can be stillborn.