Pre-eclampsia screening

Pre-eclampsia is a common pregnancy complication that is characterized by the mother developing high blood pressure (hypertension) and leaky kidneys (proteinuria). It can compromise the growth of the baby and may lead to other complications for the mother. It doesn’t usually present until at least 20 weeks of pregnancy, but often not until the third trimester.

The exact cause of pre-eclampsia isn’t fully understood, but it’s thought to involve how the blood vessels bed down in the placenta in the early stages of pregnancy.

If pre-eclampsia is diagnosed, doctors usually recommend close monitoring, and in some cases, if the condition becomes severe, early delivery of the baby might be necessary.

Symptoms and signs of pre-eclampsia

Routine antenatal checks are designed to identify pre-eclampsia by looking out for the common signs, typically:

• High blood pressure (140/90 mm Hg or higher)
• Protein in the urine (which can be detected with the urine dipstick test)
• Swelling in the hands, feet, or face
• Sudden weight gain (over a short period of time)

Other symptoms that the mother may report in between antenatal checks include:

• Severe headaches
• Changes in vision (such as blurred vision or seeing spots)
• Upper abdominal pain

Very occasionally the condition can develop into a more severe form called eclampsia, which involves seizures and can be life-threatening.

Pre-eclampsia may compromise the growth of the baby, causing the baby to be smaller than expected (fetal growth restriction). This may be picked up at an ultrasound scan or if the measurement of the mother’s abdomen (the SFH, symphysis fundal height) is smaller than expected at a routine antenatal check.

Screening for pre-eclampsia

Traditionally a mother has been considered at higher risk of developing pre-eclampsia if any of the following apply:

• Over 40 years of age
• Pre-pregnancy (essential) hypertension
• Previous pregnancy affected by pre-eclampsia or a previous baby was small
• A strong family history of pre-eclampsia (mother or sister affected)
• A prior diagnosis of an auto-immune condition such as SLE (systemic lupus erythematosus)

But this method of screening picks up less than half of the mothers who will go on to develop pre-eclampsia later in their pregnancy. A more effective method of screening is to include factors to quantify placental function, including the resistance in the vessels that feed the uterus – the uterine arteries.

An abnormally high pressure in the uterine arteries suggests that there is high pressure downstream of the uterine arteries, in the placenta. It is most predictive of potential problems when measured at around 24 weeks of pregnancy but this is too late to start any preventative treatment so it is beneficial to bring screening into the first trimester in order that treatment can start when it is most effective (ideally at around 12 weeks and certainly before 16 weeks).

However, the disadvantage of screening earlier is that there is potential to worry the mother unnecessarily. The uterine arteries are naturally quite tight when the mother isn’t pregnant as the uterus is only small and doesn’t need a particularly big blood supply. As the pregnancy progresses these vessels dilate up to provide the uterus with much more blood. The more elastic these vessels are, the easier they will dilate up and the lower the resistance. Sometimes the vessels are still a bit tight in the first trimester, but over time they become more elastic and the pressure falls. The risk of pre-eclampsia is highest in those in whom the pressure remains high. But the ideal is to start treatment at 12 weeks, and certainly before 16 weeks, so it is advisable to screen at the same time as the nuchal translucency scan (between 11 weeks and 3 days and 14 weeks). But do be aware that not everyone who is found to be high risk at this early stage will actually go on to develop pre-eclampsia.

The screening is most effective if it also takes into account aspects of the mother’s history, including her age; whether this is the first pregnancy; if a previous pregnancy was affected by pre-eclampsia or a previous baby was small; if the mother has a strong family history of pre-eclampsia (her mother or sister developed pre-eclampsia); or whether the mother has previously been diagnosed with an auto-immune condition such as SLE (systemic lupus erythematosus). This information is then combined with the following measurable factors:

• Maternal blood pressure at the time of screening
• The pulsatility index (PI) in the uterine arteries (a measure of the resistance, averaging the PI on the left and right sides)
• The level of one or more placental hormones. The one commonly measured is the PAPP-A (pregnancy-associated plasma protein-A) as this also serves as a marker for chromosomal abnormalities so can be used in both screening tests.

The screening result is not usually available until a few days after the scan as the PAPP-A result is not always immediately available. However, once all the components have been factored in, a risk for pre-eclampsia is calculated based on the risk calculator developed by the Fetal Medicine Foundation. If the final risk is higher than 1 in 100, the mother will be considered high risk and treatment offered.

The uterine artery resistance is tested again at the 20-week scan and additional monitoring is recommended if the resistance is found to be high then. It is normal for the resistance in the uterine arteries to improve as the pregnancy advances so it would be unusual to be found at high risk at 20 weeks having been considered low risk in the first trimester, but this acts as a double check.

The screening test for pre-eclampsia will also pick up babies who do not go on to meet their full growth potential during the pregnancy (fetal growth restriction) even though the mother doesn’t have any obvious signs of pre-eclampsia. The pathology is though to be similar – poor placental function – and therefore the management of these pregnancies is similar once other reasons for the baby’s smallness have been excluded.

Although the screening test is effective, it will not identify every mother who goes on to develop pre-eclampsia or fetal growth restriction, so all mothers should be aware of the symptoms and signs and attend their routine antenatal checks, even if they have been told that they are low risk.

Management options

After years of research trying to find treatments to prevent pre-eclampsia, the only real intervention seems to be very simple – a small dose of aspirin (150mg) taken at night. It appears to work by helping to open up the placental vessels and improve the placental circulation. But it works best if started at around 12 weeks, and certainly before 16 weeks. It should be continued until 36 weeks, unless a medical professional advises that it should be stopped sooner.

There are very few contra-indications to taking aspirin, although caution should be taken in those who are asthmatic or have previously had stomach ulcers. And there are no risks to the baby in taking this small dose.

Otherwise, there are no proven treatments, other than delivering the baby and the doctors managing the pregnancy will recommend this if the mother’s symptoms become so severe that her health is at risk or if the baby is showing signs of compromise. So, the mainstay of management is very careful monitoring of both the maternal condition and the growth and wellbeing of the baby.

What to do if you are told you are at high risk of pre-eclampsia after the first trimester scan

Being told you are high risk is always very frightening, particularly if this comes as a phone call out of the blue but please remember:

• Not everyone who is high risk will go on develop pre-eclampsia – this is a screening test and in order to identify and treat those who do go on to develop pre-eclampsia, the test will also identify some mothers who do not
• The treatment (aspirin) is safe for the baby and should be started as soon possible and certainly before 16 weeks (unless aspirin is contraindicated)
• Attend all routine antenatal checks, as well as any additional checks and scans that are recommended
• Those who are high risk after the first trimester scan and / or have high uterine artery pressure at the 20 week scan should expect additional antenatal checks and scans to monitor the growth and wellbeing of the baby
• Be aware of the symptoms and signs of pre-eclampsia and inform a health care professional or contact MAU if you are concerned.