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Preeclampsia is an illness that happens only in pregnancy and can affect both you and your baby. It starts with a problem in the placenta, which is the organ that joins you and your baby in your womb.

Getting high blood pressure while you're pregnant can be an early sign of preeclampsia.

For many women, preeclampsia is so mild that they don't even know they have it. But it can get bad enough to make you and your baby seriously ill. Preeclampsia doesn't get better until after your baby is born. So you may need to have your baby early.

Pregnancy can be stressful, so if you have preeclampsia as well it can be frightening. But with specialist care, most women have healthy babies and get better very quickly afterward. And most women who get preeclampsia have normal pregnancies next time.

• Preeclampsia is a serious illness that you can get when you are pregnant.
• Doctors think it starts when the organ that joins you and your baby in the womb doesn't grow as it should. That organ is called the placenta.
• Because of this problem with the placenta, your baby may not get enough blood to grow properly.
• Also, harmful chemicals from the placenta can get back into your bloodstream and make your blood pressure high. You can have problems (complications) from this.
• Most women with preeclampsia don't feel sick. So you need to go to all of your regular checkups when you are pregnant, to pick it up early.
• If you get severe preeclampsia, you should stay in the hospital until your baby is born. You may need to have your baby early.
• Having preeclampsia once doesn't mean you will get it again. Many women who've had preeclampsia have a normal pregnancy the next time.

The "pre" in preeclampsia means before. The "eclampsia" means seizure. So, preeclampsia refers to the illness women get before it becomes so serious that they have a seizure. But the name is confusing because very few women with preeclampsia ever have a seizure. That's because doctors treat the illness or deliver the baby early to avoid any problems.

To understand what goes wrong in preeclampsia, it helps to know something about what happens in a normal pregnancy.

Let's look at how the placenta grows in your womb. That's where things seem to go wrong in preeclampsia.

The placenta takes nutrients and oxygen from your blood to nourish your baby in your womb. So for a healthy pregnancy, the placenta needs a good supply of blood from you.¹

Here are some of the changes that happen in your body to give the placenta enough blood.

• During the first half of your pregnancy, blood vessels in your womb get much bigger.
• These bigger blood vessels will carry blood to the placenta. To find out more, see How the placenta grows.
• Your heart starts to pump out more blood with each heartbeat. It stays like this until your baby is born.²
• Some of the extra blood flowing through your body goes to your womb to feed your baby.

We can't be sure about what goes wrong in preeclampsia. It's a complicated illness.

But what seems to happen is that the placenta doesn't get enough blood from you. This is because the blood vessels that go to the placenta don't grow properly.

Whether your baby is affected by this, and how badly, depends on how much blood gets through.

This problem in the placenta can affect both your baby and you.

• Your baby may not grow as well as he or she should. That's because your baby can't get enough food from the placenta. For more, see Preeclampsia and your baby.
• The unhealthy placenta sends harmful chemicals back into your bloodstream. The chemicals damage the lining of your blood vessels. This can cause high blood pressure, problems with your kidneys, swelling and other problems.³ For more, see Preeclampsia and you.

No one knows what causes preeclampsia. But there are so many ideas that it has been called "the disease of theories." Probably more than one thing goes wrong.

The latest thinking is that two things have to happen together for a pregnant woman to get preeclampsia.⁴

• The woman has to have some things that increase her chances of getting preeclampsia. For example, she may have high blood pressure or diabetes or be overweight. Or she may be carrying twins or triplets. Or maybe preeclampsia runs in her family.
• The placenta doesn't grow normally in the first half of pregnancy. This means not enough blood reaches the placenta in the second half of pregnancy.

Nearly all women with preeclampsia get high blood pressure. But not all women with high blood pressure get preeclampsia. You may hear high blood pressure during pregnancy called pregnancy-induced hypertension or gestational hypertension. It is not as serious as preeclampsia.⁵ But it can turn into preeclampsia at any time. For more, see Blood pressure during pregnancy.

Preeclampsia affects different women and their babies in different ways. Here is what we know.

• It can affect you, your baby, or both you and your baby.
• It can be so mild that you don't notice it. Or it can so severe that it puts both your lives in danger.
• It usually happens in the last few weeks of pregnancy. But it can start any time from 20 weeks of pregnancy to during or even just after the birth.
• It can come on very slowly, over weeks or months. Or it can get very bad in just a few days.

We don't know for sure why some women get preeclampsia and others don't. But certain things make you more likely to get it. Things that make you more likely to get an illness are called risk factors.

There are lots of risk factors for preeclampsia. They include:⁶

• Being pregnant for the first time
• Having had preeclampsia before
• Having high blood pressure before you get pregnant
• Having health problems, such as kidney problems or a condition called rheumatoid arthritis
• Being older than 40
• Carrying two or more babies.

For more, see Risk factors for preeclampsia.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.