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Screening for Down’s syndrome in pregnancy

screening tests
 
Sophie Martin
Medically reviewed by
23rd Feb 2024

What causes Down’s syndrome?

Down’s syndrome, also called Trisomy 21 or T21, is a genetic condition that occurs as the result of an extra chromosome. Chromosomes carry genetic information and are found within the nucleus of every cell in the body. Usually a cell contains 46 chromosomes – 23 from the mother and 23 from the father. Down’s syndrome occurs when the cells contain an extra copy of chromosome 21.

How will it affect my baby?

A child born with Down’s syndrome may take longer than other children to reach developmental milestones and to learn certain skills. They have characteristic physical features and are likely to have some degree of learning disability, which can vary from mild to severe.

Health problems such as heart conditions, digestive problems and hearing and vision difficulties are also more common, although many of these can be treated.

However, with the right support and opportunities people with Down’s syndrome can lead happy and fulfilling lives. More information about the condition is available on the Down’s syndrome association website at www.downs-syndrome.org.uk.

What does screening for Down’s syndrome involve?

The combined test is a screening test that is used to predict the likelihood of your baby having Down’s syndrome and other genetic conditions. The test is offered between weeks 10 -14 of pregnancy. A blood sample is taken and a scan is given to measure the fluid at the back of the baby’s neck. Information from these tests, combined with your age, is used to work out the chance of your baby having Down’s syndrome. The combined test has an 85-90% detection rate, if you missed the combined test you will be offered the quadruple screening test which can be done between 14 and 20 weeks.

If, for some reason you miss the combined test, you will be offered the quadruple blood screening test at between 14 -20 weeks. This is not quite as accurate as the combined test.

Non-invasive prenatal tests (NIPT) are screening tests which can be carried out as a blood test taken from the mother from 10 weeks. These look at the foetal DNA in the mother's blood. NIPT detects more than 98 out of 100 babies with Down’s syndrome and are more accurate than the combined or quadruple screening tests.  NIPT is not available to all Mums through the NHS but is being offered in some parts of the UK to mums whose initial screening test shows a high chance of Down’s syndrome.

You will be offered an anomaly scan at around 18 to 21 weeks. However, as 1 in 2 babies with Down’s Syndrome will not show features of the syndrome at this stage it can’t be relied on as a method of detection.

Can screening tests harm me or my baby?

The tests will not harm either you or your baby.

What do the results mean?

If the combined or quadruple screening test shows a lower chance result this means the chance of your baby having Down’s syndrome is lower than 1 in 150. Although a lower “ chance result doesn’t mean that there is no chance, the chances of your baby being born with Down’s syndrome are very slight and therefore no further tests are offered.

If you have a high chance result from the combined or quadruple screening tests this doesn’t mean that your baby definitely has Down’s syndrome. You will be offered a diagnostic test, which is a procedure such as amniocentesis or chorionic villus sampling (CVS) that will tell you for certain whether your baby has Down’s syndrome or not. These diagnostic tests carry a small risk of miscarriage.

Your healthcare professionals will talk through your results and what they mean and you can get more information from the charity Antenatal Results and Choices (ARC) opens in a new window .

Do I have to have the test?

It is entirely up to you whether or not you have these tests. For some women it is important simply to know whether their baby has the condition as they wish to be prepared. For others, they want to know as they would not wish to continue the pregnancy if they discovered their baby had the condition.

You can find out more information about screening tests in pregnancy from the Government website opens in a new tab/window . .

What if my baby has Down’s syndrome?

If a diagnostic test shows that your baby has Down’s syndrome you will be offered time and opportunity to discuss the results and your options with your health care professional. They are there to advise and support you whatever decision you make. You can also find more information from PADS opens in a new tab/window  around Down's Syndrome.

 

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