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Testing for Down Syndrome during pregnancy

You will be offered the chance to screen for Down’s, Edwards’ and Platau’s Syndrome during your pregnancy on the NHS, but how is it done and is it safe?

Screening for Down's Syndrome

Understanding the Down's Syndrome test in pregnancy

Downs syndrome

Everyone hopes that their baby will be born in full health, but with every pregnancy there is a chance that your baby may be impacted by Down Syndrome, or other conditions such as Edwards’ or Platau Syndrome, which are caused by chromosome abnormalities. Every women in England will be offered the opportunity to test for these conditions in their unborn baby, but is it conclusive, and is it safe? We cover these questions and more below.

What is Down’s Syndrome/Edwards’ Syndrome/Patau’s Syndrome?

As you may remember from school science classes, the bodies’ genetic structure is made up of cells, which each contain 23 pairs of chromosomes. Down Syndrome, Edwards’ Syndrome and Patau’s Syndrome are conditions caused when the egg and sperm fuse, but an extra chromosome is created, meaning there are three of that chromosome in every cell in the body.

An additional number 21 chromosome causes Down Syndrome (or Trisomy 21/T21), while Edwards’ Syndrome is caused by an extra number 18 (T18) and Patau’s Syndrome by a third number 13 (T13).

Down’s Syndrome

Down Syndrome is widely recognised, characterised by distinctive facial features and causing learning difficulties, meaning those affected find it harder to learn new things or understand people. The condition varies in severity, with some children only mildly affected, and many able to attend mainstream primary school, going on to work and live relatively autonomously, with a life expectancy of around 60 years. 

While there is a chance of Down Syndrome with any pregnancy, the odds increase the older the mother is. The NHS state that a mother who is:
20 years of age has a chance of one in 1,500
30 years of age has a chance of one in 800
35 years of age has a chance of one in 270
40 years of age has a chance of one in 100
45 years of age has a chance of one in 50 or greater

Edwards’ Syndrome/Patau’s Syndrome

Sadly the effects of Edwards’ and Patau’s Syndrome are much more severe, with most babies dying before birth or shortly afterwards. While some babies do live to adulthood, this is uncommon. The conditions cause a wide range of serious health problems, with Edwards’ Syndrome resulting in unusual head and facial features, growth problems and children who are unable to stand or walk, while Patau’s syndrome causes heart problems, cleft lip and palates, poorly formed eyes and ears, kidney problems and inability to stand/walk. 

Thankfully these conditions are rare, with Edwards’ Syndrome affecting just 3 in 10,000 births, while Patau’s Syndrome affects 2 in 10,000. 

Screening for Down’s/Edwards’/Patau’s during pregnancy

Screening at 10-14 weeks pregnant

All women in England are offered the chance to test for Downs Syndrome, Edwards’ Syndrome and Patau’s Syndrome during their pregnancy. This will usually be done with a combined test (ultrasound and blood test) in weeks 10 - 14 weeks of pregnancy. The ultrasound test/nuchal translucency (NT) scan checks the amount of fluid behind your baby’s neck, while the blood test samples the proteins human chorionic gonadotrophin (hCG) and pregnancy-associated plasma protein (PAPP-A) in your blood. 

Screening at 14-20 weeks pregnant

If your pregnancy is too far along for the combined test you will be offered other options. For Down’s Syndrome the quadruple test (a blood test which samples four proteins) is available in weeks 14 to 20, while abnormalities caused by Edwards’ and Patau’s Syndrome can be checked for at your mid-pregnancy scan.

Is screening essential?

Whether you wish to go ahead with the screening is up to you, and should be considered carefully together with your partner. You can opt to only be tested for Down’s, only Edwards’ and Patau’s, both or neither. None of the tests above carry any risk of hurting your baby or miscarriage.

If you choose not to be tested you can still have your dating ultrasound scan, but you should be aware that abnormalities may be picked up during the scan, and you will be told if they’re discovered. 

Understanding your test results

Once you’ve been screened, the results of your test combined with your age will be used to determine the likelihood that your baby could be affected by Down’s/Edwards’/Patau’s Syndrome. 

This test is not conclusive, but will give you the odds of your baby being affected. The results will be characterised as ‘high risk’ or ‘low risk’, with high risk being a 1 in 20 chance, and low risk as a less than 1 in 150 chance. You will receive two separate risk results one for Down’s and another combined result for Edwards’ and Patau’s.

If your results are high risk you will usually be notified within 3 working days, while low risk results should be returned within 2 weeks. However this can depend on your local area, so check with your midwife for details.

What happens if the result is ‘high risk’?

If your results are categorised as high risk, this doesn’t mean your baby is definitely affected. You will be offered further diagnostic tests to confirm whether the Syndrome is present. Currently you can have a non-invasive test (NIPT) which takes a further blood sample and provides a result with 98% accuracy. This offers no risk of miscarriage, but you may have to pay to have this done privately. 

Alternatively the NHS offers two types of test; amniocentesis, where a fine needle is used to take a sample of amniotic fluid, or chorionic villus sampling (CSV) where a fine needle is used to take a sample of the placenta. These tests carry a one in 100 chance of miscarriage, but they are conclusive, so should be considered careful before going ahead.

If your result is high risk, whether or not you choose to gain a conclusive result, the charity ARC (antenatal results and choices) can help support you through your journey, and also has a forum where you can speak to other parents in the same situation. 

Testing for Down Syndrome during pregnancy