Screening tests for these three conditions are non-invasive tests which are offered to all pregnant women in the UK (except Northern Ireland). They are not diagnostic and so cannot give a yes or no answer. Instead, they help to identify women whose chance of having a baby with the syndromes is greater than 1 in 150. They consist of an ultrasound scan and a blood test or just a blood test.
You do not have to have screening tests for Down's, Edwards and Patau's syndromes. Your decision to go ahead might be influenced by how you feel about having a baby with these conditions and how important it is for you to know whether your baby has one of these syndromes before he or she is born. You can choose have screening for all three conditions, just Edwards and Patau’s or just Down’s syndrome.
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.
Results are most often reported as a statistical chance and sometimes the terms "increased chance" or "low chance" will be used.
The terms "risk" and "chance" refer to the possibility of an event happening. For example, a chance of 1 in 100 means that out of 100 women with this result, 1 will have a baby with a syndrome and 99 will not. This is the same as a 1% chance that the baby has a syndrome and a 99% chance that the baby does not.
|Chance of a 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%|
All hospitals have a number they use as a cut-off between those results that are categorised as "increased chance" (further testing is offered) and "low chance" (no further testing is offered). This number is usually 1 in 150. This means that all women who have a result between 1 in 2 and 1 in 150 will be offered a diagnostic test such as CVS or amniocentesis.
Most women with an increased chance result will not have a baby with one of the syndromes. If your hospital uses a 1 in 150 cut-off, then results outside this, from 1 in 151 and beyond are classified as "low chance". It is important to remember that this does not mean your baby definitely does not have a syndrome, it means it is very unlikely.
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 chance" result which is a result giving a chance of between 1 in 2 and 1 in 149 of the baby having Down's syndrome.
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For more information about fetal anomaly screening in England:
For more information about Edwards and Patau's syndromes: