Down’s Syndrome (also referred to as Trisomy 21) occurs when there is a change in one of the genes in the egg before prior to fertilisation. The change, which results in the embryo developing with 3 copies of chromosome 21, instead of the normal 2, is a random event which occurs with greater frequency as the age of the mother increases. For example, a woman of 40 is 10 times more likely to have a Down’s Syndrome pregnancy than a 23 year old and by the time a woman is 45 she has a 1 in 40 risk of bearing a baby with Down’s Syndrome.

The new test for Down’s Syndrome is a blood test which looks at DNA and is highly accurate in detecting whether the baby has an extra copy of chromosome 21. Details published in the BMJ (British Medical Journal) looked at 753 woman with a high risk of having a Down’s Syndrome baby and found that 86 of them carried a baby with a chromosomal disorder. The results showed that the test was highly accurate and had not missed any cases. If this test was used with woman who would normally be recommended to have either amniocentesis or chronic villus testing, then the need for these procedures could be reduced by ruling out Down’s Syndrome in 98% of these cases leaving only 0.1% of women needing one of the more invasive tests.

It should be noted that although the test does not give false negatives, there is still a chance of a false positive result. Also the test may not be able for some time due to cost and regulatory issues.

The first step in the exisitng screening process is a blood test which looks at the levels of certain proteins and hormones in the mother’s blood stream. The results of the test give a risk assessment that is more accurate than the risk based on age alone, for example the result of 1 in 500 would mean that of 500 pregnancies only 1 would have an affected baby. Alternatively, results may be given in the form of a positive or negative result, based on a pre-determined cut off such as 1 in 250, whereby if the risk of an affected pregnancy is 1 in 250 or greater then a positive result is given with the recommendation that further screening be considered.

In addition to the blood test, ultrasound scan examination of nuchal translucency can help to identify whether a baby may be affected by Down’s Syndrome. This scan focuses on the area at the nape of the baby’s neck, as babies with Down’s Syndrome to have more fluid there than normal so measuring the thickness of fluid can help to identify whether a baby is likely to have Down’s Syndrome.

Ideally, both the ultrasound and the blood test should be performed by about 13 weeks, although testing can still be carried out later in pregnancy. Depending upon the results of these tests women may be advised to consider further testing. It is at this point that the new test would be introduced helping to reduce the need for what are currently the next steps:

In CVS, which can be performed after the 10th week of pregnancy,  a small sample of the placenta is taken and sent to the lab for testing. In amniocentesis, typically between 15-22 weeks, a sample of amniotic fluid is taken for analysis. Although both these test give more accurate results than the ultrasound and blood tests, they can still provide either false positive (up to about 5%) or false negative (between 10-15%) results and may give rise to complications such as infection and miscarriage (of which there is about a 1% risk).

The new screening test for Down’s Syndrome would therefore offer more accurate results, with less risk and less discomfort to the pregnant mothers.