Down's syndrome testing in pregnancy

What happens with Down’s syndrome testing? We break it down so you know what’s what.

What is Down’s syndrome?

Down's syndrome is also known as Down syndrome or trisomy 21. Down’s syndrome is a genetic condition that causes learning disabilities and specific physical characteristics (NHS Choices, 2017). They might include:

  • eyes that slant upwards and outwards 
  • floppiness (hypotonia – decreased muscle tone)
  • a small mouth with a tongue that may stick out 
  • flat back of the head 
  • below-average weight and length at birth 
  • palm with only one crease across it. 

(NHS Choices, 2017)

While the average IQ in the UK is 100, in children with Down's syndrome it’s around 50 and ranges from 30 to 70 (Fishler and Kohl, 1991; Mégarbané et al, 2013). Down’s syndrome has also been associated with altered immune system functions, respiratory and gastrointestinal infections, congenital heart disease, hypothyroidism and hearing loss (Malt et al, 2013).

What causes Down’s syndrome?

An extra chromosome 21 causes most cases of Down’s syndrome. It can also be caused by a part of a chromosome transferring to another one (NIH National Human Genome Research Institute, 2016; NDSS, 2018).

"The risk of Down’s syndrome is linked to the age you are when you get pregnant. If you’re 35 years old, you have about a one in 350 chance of conceiving a child with Down’s syndrome. At age 40 it is 1 in 100 and by age 45, it’s 1 in 30 (NDSS, 2018)."

Down’s syndrome screening tests – the combined test

All pregnant women in England are offered a screening test called the combined test between 10 and 14 weeks of pregnancy (NHS Choices, 2018a). This test combines an ultrasound scan with a blood test for Down's syndrome, Edwards' syndrome and Patau's syndrome. It’ll tell you how high your chances are of having a baby with one of those conditions (NHS Choices, 2018a).

At the test, the fluid at the back of the baby's neck will be measured to determine the ‘nuchal translucency’. Nuchal translucency is the name for the fluid under the skin at the back of the baby’s neck.  As the NT increases, the risk of Down’s syndrome increases (NHS Choices, 2018a).

You will also have blood taken to check the levels of two pregnancy hormones. These two tests will then be put together with your age to work out the chance of your baby having Down's, Edwards' or Patau's syndromes (Basingstoke and North Hampshire NHS Trust, 2010; Shiefa et al, 2013; Basingstoke and North Hampshire NHS Foundation Trust, 2016; Insider Radiology, 2017).  

Down’s syndrome screening tests – the quadruple test

Getting a nuchal translucency measurement depends on the position of the baby so it’s not always possible to do it. If it’s tricky in your case, you’ll be offered a different blood screening test, called the quadruple test, which isn’t as accurate as the combined test (NHS Choices, 2018a).

Down’s syndrome screening tests – results

If your screening test returns a lower-chance result, the chances of your baby having Down’s syndrome are less than one in 150 (NHS Choices, 2018a).

If you have a higher-chance result, you will be offered a diagnostic test, such as amniocentesis or chorionic villus sampling (CVS). This will tell you for certain whether or not the baby has Down's, Edwards' or Patau's syndrome (Health Knowledge, 2017; NHS Choices, 2018a).

Down’s syndrome tests – NIPT or Harmony test

If your screening test shows a higher chance, you can also take an NIPT for Down syndrome test or Harmony test. Currently, NIPT isn’t available through the NHS but private health care providers can do it for you (ARC, 2018; The NHS Rapid Project, 2018). See the Antenatal Results and Choices website for more information.

The NIPT or Harmony test involves a blood sample. If it indicates Down’s syndrome, an invasive test will be recommended to confirm the result. NIPT has an accuracy rate of 98%, and it can usually be done by the time you’re 10 weeks pregnant (ARC, 2018; The NHS Rapid Project, 2018).

Down’s syndrome tests – the diagnostic test (amniocentesis or chorionic villus)

A diagnostic test like amniocentesis or chorionic villus sampling (CVS) will tell you for certain whether your baby has Down's, Edward’s or Patau's syndrome.

Amniocentesis involves removing and testing cells from the fluid that surrounds your unborn baby in the womb with a long, thin needle. Women usually have an amniocentesis between weeks 15 and 20 weeks of pregnancy but possibly later if necessary (NHS Choices, 2016).

With chorionic villus sampling, a small sample of cells is removed from the placenta. This is either by a needle inserted through your tummy or a tube or small forceps that go through the cervix. Women usually have a chorionic villus sampling test between weeks 11 and 14 weeks of pregnancy, or later if necessary (NHS Choices, 2018b).

About 0.5 to 1 in 100 amniocentesis or chorionic villus sampling tests result in a miscarriage so it’s a big decision whether to have them (NHS Choices, 2018b). The charity Antenatal Results and Choices offers lots of information about screening results and your options.

Down’s syndrome diagnosis: Support

If you have received a diagnosis for Down’s syndrome based on one of the diagnostic tests, it is important to try to get as much information as possible. You could take a look at the following:

For a list of local support groups for parents, check out the Down Syndrome Development Trust.

This page was last reviewed in March 2018.

Further information

Our support line offers practical and emotional support with feeding your baby and general enquiries for parents, members and volunteers: 0300 330 0700.

We also offer antenatal courses which are a great way to find out more about birth, labour and life with a new baby.

Make friends with other parents-to-be and new parents in your local area for support and friendship by seeing what NCT activities are happening nearby.

Basingstoke and North Hampshire NHS Foundation Trust. (2016)  Low PAPP-A Information for parents. Available from:  https://www.hampshirehospitals.nhs.uk/media/423511/low_papp-a.pdf [Accessed: 27th March 2018]

Fishler K, Koch R. (1991) Mental development in Down syndrome mosaicism. American Journal on Mental Retardation 96(3):345-351. Available from:  https://www.ncbi.nlm.nih.gov/pubmed/1836734 [Accessed: 27th March 2018]

Health Knowledge (2017) Differences between screening and diagnostic tests and case finding. Available from: https://www.healthknowledge.org.uk/public-health-textbook/disease-causation-diagnostic/2c-diagnosis-screening/screening-diagnostic-case-finding [Accessed: 27th March 2018]

Inside Radiology. (2018) Nuclear translucency scan. Available from: https://www.insideradiology.com.au/nuchal-translucency-scan/ [Accessed: 27th March 2018]

Malt EA, Dahl RC, Haugsand TM, Ulvestad IH, Emilsen NM, Hansen B, Davidsen EM. (2013) Health and disease in adults with Down syndrome.  133(3):290-294. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23381164 [Accessed: 27th March 2018]

Mégarbané A, Noguier F, Stora S, Manchon L, Mircher C, Bruno R, Ravel,A. (2013). The intellectual disability of trisomy 21: differences in gene expression in a case series of patients with lower and higher IQ. European Journal of Human Genetics 21(11):1253. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23422941 [Accessed: 27th March 2018]

NDSS (National Down Syndrome Society). (2018) Down syndrome. Available from:  https://www.ndss.org/about-down-syndrome/down-syndrome/ [Accessed: 27th March 2018]

NHS Choices. (2016) Amniocentesis. Available  from:  https://www.nhs.uk/conditions/amniocentesis/ [Accessed: 27th March 2018]

NHS Choices. (2017) Down’s syndrome. Available from: https://www.nhs.uk/conditions/downs-syndrome/ [Accessed: 27th March 2018]

NHS Choices. (2018a) Screening for Down’s, Edwards’ and Patau’s syndrome. Available from: https://www.nhs.uk/conditions/pregnancy-and-baby/screening-amniocentesis-downs-syndrome/ [Accessed: 27th March 2018]

NHS Choices. (2018b) Chorionic villus sampling. Available from:  https://www.nhs.uk/conditions/chorionic-villus-sampling-cvs/  [Accessed: 27th March 2018]

NIH National Human Genome Research Institute. (2016) Chromosome abnormalities. Available from: https://www.genome.gov/11508982/chromosome-abnormalities-fact-sheet/#al-5 [Accessed: 27th March 2018]

Shiefa S, Amargandhi M, Bhupendra J, Moulali S, Kristine T. (2013). First trimester maternal serum screening using biochemical markers PAPP-A and free β-hCG for down syndrome, patau syndrome and edward syndrome. Indian Journal of Clinical Biochemistry 28(1):3-12. Available from: https://www.ncbi.nlm.nih.gov/pubmed/24381414 [Accessed: 27th March 2018]

The NHS Rapid Project. (2018) NIPT for Down syndrome: A guide for patients and healthcare professionals. Available from: http://www.rapid.nhs.uk/guides-to-nipd-nipt/nipt-for-down-syndrome/ [Accessed: 27th March 2018]

Further reading

American Pregnancy Association (2016) Quad screen test. Available from:  http://americanpregnancy.org/prenatal-testing/quad-screen/ [Accessed: 27th March 2018]

Antenatal Results and Choices (ARC) (2018) Screening for Down’s, Edwards and Patau’s Syndrome/Non-invasive Pre-natal Testing (NIPT). Available from: http://www.arc-uk.org/tests-explained/non-invasive-prenatal-testing-nipt [Accessed: 27th March 2018]

Basingstoke and North Hampshire NHS Foundation Trust. (2016)  HCG information leaflet. Available from: http://www.hampshirehospitals.nhs.uk/uploaded_files/GP%20section/EPAU/hcg_info_leaflet_march2010.pdf [Accessed: 27th March 2018]

NHS Choices. (2016)  Mid-pregnancy anomaly scan. Available from: https://www.nhs.uk/conditions/pregnancy-and-baby/anomaly-scan-18-19-20-21-weeks-pregnant/ [Accessed: 27th March 2018]

NHS Choices. (2017) The pregnancy dating scan. Available from: https://www.nhs.uk/conditions/pregnancy-and-baby/dating-scan-ultrasound-10-11-12-13-weeks-pregnant/ [Accessed: 27th March 2018]

NHS Choices. (2018) Screening tests in pregnancy. Available from:  https://www.nhs.uk/conditions/pregnancy-and-baby/screening-tests-abnormality-pregnant/ [Accessed: 27th March 2018]

NIH U.S. National Library of Medicine. Health Conditions -Down syndrome https://ghr.nlm.nih.gov/condition/down-syndrome#inheritance [Accessed: 27th March 2018]

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