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Down's Syndrome Screening Tests


Introduction Show [+]Hide [-]

This information explains which screening tests for Down’s syndrome are available at the RVI maternity unit. More information is available in the leaflet “Screening tests for you and your baby” (National Screening Committee 2008) which you can obtain from your midwife. She will discuss these tests with you in more detail so that you can decide whether you would like a screening test for Down’s syndrome or not.  

Please be aware that a screening test will not tell you if your baby does or does not have Down’s syndrome but will tell you the chance (or risk) of your baby having Down’s syndrome.

Before 13 weeks of pregnancy Show [+]Hide [-]

If you book with your community midwife before 13 weeks of pregnancy you will be offered the Combined Test (CT). This involves attending the ultrasound department for a scan to measure the skin thickness at the back of the baby’s neck (the nuchal translucency or NT) at around 12 weeks of pregnancy. This is followed by taking a blood sample which is sent to the laboratory where two substances (proteins) are measured. These proteins are slightly different in women carrying a baby with Down’s syndrome. The laboratory combines the results from the blood test and the scan to give you the risk of your baby having Down’s syndrome. A result cannot be given unless both the scan and the blood test are completed.

The combined test aims to detect nine out of 10 (90%) Down’s syndrome affected pregnancies. This is called the detection rate. However two to three in a 100 women (2-3%) will be told they are at increased risk of Down’s syndrome when actually their baby is unaffected. This is called the false positive rate.  

If the result is low risk from this combined test then you will get the result by letter within two weeks of having the tests and no further tests will be required. You will still be offered the 20 week scan.   

If you have a risk that is one in 150 (0.7%) or higher you will be contacted by a specialist midwife (usually by telephone) within three working days to discuss the result so you can decide whether you would like a further test to find out for certain whether your baby has Down’s syndrome or not.

Between 14 and 20 weeks of pregnancy Show [+]Hide [-]

If you book with your midwife between 14 and 20 weeks of pregnancy you will be offered the “Quadruple Test”. This involves your midwife taking a blood sample which can be taken from 15 to 20 weeks of pregnancy but ideally at around 16 weeks. You will be offered a scan before the blood test so we know exactly how many weeks pregnant you are. Four substances (proteins) in your blood are measured in the laboratory and these are used to calculate the risk of your baby having Down’s syndrome. A result will not be given unless both the scan and the blood test are completed.

The quadruple test is accurate in eight out of 10 (80%)  Down’s syndrome affected pregnancies.  This is called the detection rate.  Four to five in a 100 women (4-5%) will be told they are at increased risk when actually their baby is unaffected.  This is called a false positive rate.

If you have a low risk result from this blood test then you will get the result by letter within two weeks of having the test and no further tests will be required. You will still be offered the 20 week scan.

If you have a risk that is one in 200 (0.5%) or higher you will be contacted by a specialist midwife (usually by telephone) within three working days to discuss the result so you can decide whether you would like a further test to find out for certain whether your baby has Down’s syndrome or not.

For further information please ask your community midwife.

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