Pre-eclampsia rarely happens before the 20th week of pregnancy. Most cases occur after 24 to 26 weeks, and usually towards the end of pregnancy.

Although less common, the condition can also develop for the first time in the first 6 weeks after birth.

Most people only experience mild symptoms, but it's important to manage the condition in case severe symptoms or complications develop.

Generally, the earlier pre-eclampsia develops, the more severe the condition will be.

Early signs and symptoms

Initially, pre-eclampsia causes:

• high blood pressure (hypertension)
• protein in urine (proteinuria)

You probably won't notice any symptoms of either of these, but your GP or midwife should pick them up during your routine antenatal appointments.

High blood pressure affects 10 to 15% of all pregnant women, so this alone doesn't suggest pre-eclampsia.

But if protein in the urine is found at the same time as high blood pressure, it's a good indicator of the condition.

Read more about diagnosing pre-eclampsia.

Further symptoms

As pre-eclampsia progresses, it may cause:

• severe headaches
• vision problems, such as blurring or seeing flashing lights
• severe heartburn
• pain just below the ribs
• nausea or vomiting
• excessive weight gain caused by fluid retention
• feeling very unwell
• sudden increase in oedema – swelling of the feet, ankles, face and hands

If you notice any symptoms of pre-eclampsia, get medical advice immediately by calling your GP surgery or NHS 111.

Without immediate treatment, pre-eclampsia may lead to a number of serious complications, including:

• convulsions (eclampsia)
• HELLP syndrome (a combined liver and blood clotting disorder)
• stroke

But these complications are rare.

Read more about the complications of pre-eclampsia.

Signs in the unborn baby

The main sign of pre-eclampsia in the unborn baby is slow growth. This is caused by poor blood supply through the placenta to the baby.

The growing baby receives less oxygen and fewer nutrients than it should, which can affect development. This is called intra-uterine or foetal growth restriction.

If your baby is growing more slowly than usual, this will normally be picked up during your antenatal appointments, when the midwife or doctor measures you.

Pre-eclampsia is thought to be caused by the placenta not developing properly due to a problem with the blood vessels supplying it. The exact cause isn't fully understood.

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 doesn't get enough blood. This could be because the placenta didn't develop properly as it was forming during the first half of the pregnancy.

The problem with the placenta means the blood supply between mother and baby is disrupted.

Signals or substances 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 important 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 the 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 eventually grow into the placenta.

During the early stages of pregnancy, these blood vessels change shape and become wider.

If the blood vessels don't fully transform, it's likely that the placenta won't develop properly because it won't get enough nutrients. This may lead to pre-eclampsia.

It's still unclear why the blood vessels don't transform as they should. 

It's likely that inherited changes in your genes have some sort of role, as the condition often runs in families. But this only explains some cases.

Who's most at risk?

Some factors have been identified that could increase your chances of developing pre-eclampsia.

These include:

• having an existing medical problem – such as diabetes, kidney disease, high blood pressure, lupus or antiphospholipid syndrome
• previously having pre-eclampsia – there's an approximately 16% chance you'll develop the condition again in later pregnancies

Some factors also increase your chances by a small amount.

If you have 2 or more of these together, your chances are higher:

• it's your first pregnancy – pre-eclampsia is more likely to happen during the first pregnancy than during any subsequent pregnancies
• it's 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're over the age of 40
• you were obese at the start of your pregnancy – meaning you had a body mass index (BMI) of 35 or more
• you're expecting multiple babies, such as twins or triplets

If you're 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're 12 weeks pregnant until your baby is born.

Evidence suggests this can lower your chances of developing the condition.

Pre-eclampsia is easily diagnosed during the routine checks you have while you're pregnant.

During these antenatal appointments, your blood pressure is regularly checked for signs of high blood pressure and a urine sample is tested to see if it contains protein.

If you notice any of the symptoms of pre-eclampsia between your antenatal appointments, see your midwife or GP for advice.

Blood pressure

Blood pressure is a measure of the force of blood on artery walls (main blood vessels) as it flows through them.

It's measured in millimetres of mercury (mmHg) and recorded as 2 figures:

• systolic pressure – the pressure when the heart beats and squeezes blood out
• diastolic pressure – the pressure when the heart rests inbetween beats

Your GP or midwife will use a device with an inflatable cuff and a scale as a pressure gauge (a sphygmomanometer) to measure your blood pressure.

The systolic reading is taken first, followed by the diastolic reading.

If, for example, the systolic blood pressure is 120mmHg and the diastolic blood pressure is 80mmHg, the overall blood pressure will be 120 over 80, which is commonly written as 120/80.

High blood pressure during pregnancy is usually defined as a systolic reading of 140mmHg or more, or a diastolic reading of 90mmHg or more.

Urine tests

A urine sample is usually requested at every antenatal appointment. This can easily be tested for protein using a dipstick. 

A dipstick is a strip of paper that's been treated with chemicals so it reacts to protein, usually by changing colour.

If the dipstick tests positive for protein, your GP or midwife may ask for another urine sample to send to a laboratory for further tests.

This could be a single sample of urine, or you may be asked to provide several samples over a 24-hour period.

These can be used to determine exactly how much protein is being lost through your urine.

Blood test

If you're between 20 weeks and 34 weeks plus 6 days pregnant and your doctors think you may have pre-eclampsia, they may offer you a blood test to help rule out pre-eclampsia.

It measures levels of a protein called placental growth factor (PIGF). If your PIGF levels are high, it's highly likely that you don't have pre-eclampsia.

If your PIGF levels are low, it could be a sign of pre-eclampsia, but further tests would be needed to confirm the diagnosis.

Further tests in hospital

If you're diagnosed with pre-eclampsia, you should be referred to a specialist in hospital for further tests and more frequent monitoring.

Depending on the severity of your condition, you may be able to go home after an initial assessment and have frequent outpatient appointments.

In severe cases, you may need to stay in hospital for closer observation.

Read more about treating pre-eclampsia.

Pre-eclampsia can only be cured by delivering the baby. If you have pre-eclampsia, you'll be closely monitored until it's possible to deliver the baby.

Once diagnosed, you'll be referred to a hospital specialist for further assessment and any necessary treatment.

If you only have high blood pressure without any signs of pre-eclampsia, you can usually return home afterwards and attend regular (possibly daily) follow-up appointments.

If pre-eclampsia is confirmed, you'll usually need to stay in hospital until your baby can be delivered.

Monitoring in hospital

While you're in hospital, you and your baby will be monitored by:

• having regular blood pressure checks to identify any abnormal increases
• having regular urine samples taken to measure protein levels
• having various blood tests – for example, to check your kidney and liver health
• having ultrasound scans to check blood flow through the placenta, measure the growth of the baby, and observe the baby's breathing and movements
• electronically monitoring the baby's heart rate using a process called cardiotocography, which can detect any stress or distress in the baby

Medication for high blood pressure

Medication is recommended to help lower your blood pressure. These medications reduce the likelihood of serious complications, such as stroke.

Some of the medications used regularly in the UK include labetalol, nifedipine or methyldopa.

Of these medications, only labetalol is specifically licensed for use in pregnant women with high blood pressure.

This means the medication has undergone clinical trials that have found it to be safe and effective for this purpose.

But while methyldopa and nifedipine aren't licensed for use in pregnancy, they can be used "off-label" (outside their licence) if it's felt the benefits of treatment are likely to outweigh the risks of harm to you or your baby.

These medications have been used by doctors in the UK for many years to treat pregnant women with high blood pressure.

They're recommended as possible alternatives to labetalol in guidelines produced by the National Institute for Health and Care Excellence (NICE).

Your doctors may recommend one of them if they think it's the most suitable medication for you. 

If your doctors recommend treatment with one of these medications, you should be made aware that the medication is unlicensed in pregnancy and any risks should be explained before you agree to treatment, unless immediate treatment is needed in an emergency.

Other medications

Anticonvulsant medication may be prescribed to prevent fits if you have severe pre-eclampsia and your baby is due within 24 hours, or if you have had convulsions (fits).

They can also be used to treat fits if they occur.

Delivering your baby

In most cases of pre-eclampsia, having your baby at about the 37th to 38th week of pregnancy is recommended.

This may mean that labour needs to be started artificially (known as induced labour) or you may need to have a caesarean section.

This is recommended because research suggests there's no benefit in waiting for labour to start by itself after this point.

Delivering the baby early can also reduce the risk of complications from pre-eclampsia.

If your condition becomes more severe before 37 weeks and there are serious concerns about the health of you or your baby, earlier delivery may be necessary.

Deliveries before 37 weeks are known as premature births and babies born before this point may not be fully developed.

You should be given information about the risks of both premature birth and pre-eclampsia so the best decision can be made about your treatment.

After the delivery

Although pre-eclampsia usually improves soon after your baby is born, complications can sometimes develop a few days later.

You may need to stay in hospital after the delivery so you can be monitored.

Your baby may also need to be monitored and stay in a hospital neonatal intensive care unit if they're born prematurely.

These units have facilities that can replicate the functions of the womb and allow your baby to develop fully.

Once it's safe to do so, you'll be able to take your baby home.

You'll usually need to have your blood pressure checked regularly after leaving hospital, and you may need to continue taking medication to lower your blood pressure for several weeks.

You should be offered a postnatal appointment 6 to 8 weeks after your baby is born to check your progress and decide if any treatment needs to continue. This appointment will usually be with your GP.

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 her baby.

Problems affecting the mother

Fits (eclampsia)

Eclampsia describes a type of convulsion or fit (involuntary contraction of the muscles) that pregnant women can experience, usually from week 20 of the pregnancy or immediately after the birth.

Eclampsia is quite rare in the UK, with an estimated 1 case for every 4,000 pregnancies.

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.

Of those who have eclampsia, around 1 in 50 will die from the condition. Unborn babies can suffocate during a seizure and 1 in 14 may die.

Research has found that a medication 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

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:

• "H" is for haemolysis – this is where the red blood cells in the blood break down
• "EL" is for elevated liver enzymes (proteins) – a high number of enzymes in the liver is a sign of liver damage
• "LP" is for low platelet count – platelets are substances in the blood that help it clot

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. 

Stroke

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.

Organ problems

• pulmonary oedema – where fluid builds up in and around the lungs. This stops the lungs working properly by preventing them absorbing oxygen.
• kidney failure – when the kidneys can't filter waste products from the blood. This causes toxins and fluids to build up in the body.
• liver failure – disruption to the functions of the liver. The liver has many functions, including digesting proteins and fats, producing bile and removing toxins. Any damage that disrupts these functions could be fatal.

Blood clotting disorder

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.

Problems affecting the baby

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 of women with pre-eclampsia can even die in the womb and be stillborn.

It's estimated around 1,000 babies die each year because of pre-eclampsia. Most of these babies die because of complications related to early delivery.