What is the nuchal test

asks Ursula:
A. The nuchal translucency scan is a relatively new antenatal screening test.

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Q. How does it work
A. Virtually all pergnant women are given routine ultrasound scans to detect abnormalities - especially in the spine and head. Now it has been found that ultrasound scans can be useful for screening for Down's syndrome. Studies have shown that the thickness of the 'nuchal fold' at the back of the baby's neck is related to the risk of Down's syndrome. An ultrasound scan is done between 10 and 13 weeks to take a measurement, from which a risk factor to be calculated.

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Q. What happens if the nuchal test indicates that you may be carrying a Down's syndrome baby
A. You would be offered a test which can tell for certain when your baby has Down's syndrome - amniocentesis - which involves inserting a fine needles into the abdomen to extract amniotic fluid for testing. However, this carries a one to three per cent risk of miscarriage.

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However, scientists believe another non-intrusive measurement may test for Down's syndrome more accurately than a nuchal scan and reduce the need for an invasive test.

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Q. What is it
A. It's also an ultrasound scan, but this time it's used to check for the presence of a bone in the nose. This is done between 11 and 14 weeks into the pregnancy, Research by Professor Kypros Nicolaides, of London's Harris Birthright Research Centre for Fetal Medicine at Kings College Hospital School of Medicine, found that absence of the bone at this stage in pregnancy is linked to Down's syndrome. Researchers looked at 700 foetuses aged between 11 and 14 weeks. Their research, published in The Lancet, showed that nasal bone was found to be absent in 73% of 59 foetuses later found to have Down's syndrome, and in just three (0.5%) of 603 chromosomally normal foetuses.

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Q. How accurate are these tests
A. A combination of the mother's age, and the blood and nuchal translucency tests, give a 75% accurate indication of Down's syndrome. If the nasal bone test was also used alongside age and the nuchal scan, the accuracy of prediction could increase to 85% or more, scientists say.

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Q. Will my hospital do a nuchal translucency scan
A. Perhaps. They're not widely available at the moment, but are becoming more so.

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Q. How many babies are born with Down's syndrome
A. About one in 1,000 babies. Older mothers have a higher chance of having a baby with Down's syndrome, although age alone is the least accurate predictor.

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By Sheena Miller