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Down's syndrome screening
Screening tests for Down’s syndrome are non-invasive tests that are offered to all pregnant women in the UK except Northern Ireland. They are not diagnostic and so cannot give a yes/no answer. Instead, they help to identify women whose chance of having a baby with Down's syndrome is greater than 1 in 150 or greater than 1 in 200, depending on the screening test. They consist of an ultrasound scan and a blood test or just a blood test.
Down’s syndrome screening in your area
Not all hospitals offer the same Down’s syndrome screening tests. Different screening tests are described on the timing of tests page and in the glossary.
If you live in England you can find out what is available at your local NHS hospital by visiting: www.fetalanomaly.screening.nhs.uk
Deciding to have screening tests for Down’s syndrome
You do not have to have a screening test for Down’s syndrome. Your decision to go ahead might be influenced by how you feel about having a baby with this condition and how important it is for you to know whether your baby has Down’s syndrome before he or she is born.
The best source of information about living with Down’s syndrome can be found at the Down’s Syndrome Association website at www.downs-syndrome.org.uk Most women will receive a reassuring result from the test but some (approximately 3-5%) will be given a result that means they will be offered a diagnostic test.
Understanding your result
Results are most often reported as a statistical chance and sometimes the terms ‘increased risk’ or ‘low risk’ will be used. The terms "risk" and "chance" refer to the possibility of an event happening. For example, a risk of 1 in 100 means that out of 100 women with this result, 1 will have a baby with Down's syndrome and 99 will not. This is the same as a 1% chance that the baby has Down's syndrome and a 99% chance that the baby does not.
| Chance of Down's syndrome | Chance of an unaffected pregnancy | ||
|---|---|---|---|
| 1 in 4 | 25% | 3 in 4 | 75% |
| 1 in 5 | 20% | 4 in 5 | 80% |
| 1 in 10 | 10% | 9 in 10 | 90% |
| 1 in 20 | 5% | 19 in 20 | 95% |
| 1 in 30 | 3% | 29 in 30 | 97% |
| 1 in 50 | 2% |
49 in 50 | 98% |
| 1 in 100 | 1% |
99 in 100 | 99% |
| 1 in 200 | 0.5% |
199 in 200 | 99.5% |
The image below depicts a 1 in 100 result. The red dot represents the one woman out of a hundred who will have a baby with Down’s syndrome.
All hospitals have a number they use as a cut-off between those results that are categorized as ‘increased risk’ (further testing is offered) and ‘low risk’ (no further testing is offered). This number is usually either 1 in 200 or 1 in 150.
If your hospital uses 1 in 150, all women who have a result between 1 in 2 and 1 in 149 will be offered a diagnostic test such as CVS or amniocentesis.
Most women with an increased risk result will not have a baby with Down’s syndrome
If your hospital uses a 1 in 150 cut off, then results outside this, from 1 in 151 and beyond are classified as ‘low risk’. It is important to remember that this does not mean your baby definitely does not have Down’s syndrome – it means it is very unlikely.
Accuracy of Down’s syndrome screening tests
Although Down’s syndrome screening tests are not 100% accurate, the screening test offered by your hospital should meet the standards set by the UK National Screening Committee.
Current guidelines say that a screening test must detect 75% of babies with Down’s syndrome with a screen positive rate of no more than 3%. The term screen positive describes those women who were given an ‘increased risk’ result.