Page last updated at 21:14 GMT, Monday, 6 October 2008 22:14 UK

'Safer' test developed for Down's

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Parents of a toddler with Down's give their views on testing

Scientists say they have developed a safer blood test that can tell a women if her unborn baby has Down's syndrome.

Invasive procedures currently used risk miscarriage and damage to the foetus.

A Stanford University test of DNA evidence in 18 pregnant women's blood correctly identified nine cases of Down's syndrome, reports PNAS journal.

The university now wants a larger-scale test. Babies with Down's syndrome have an extra copy of chromosome 21, causing physical and intellectual impairments.

Non-invasive testing will be much safer than current approaches
Researcher Dr Stephen Quake

If a foetus has three copies of chromosome 21 rather than the normal two, there will also be a relative increase in the quantity of chromosome 21 in the mother's blood because DNA can cross the placenta from the baby to the mother.

The "shotgun sequencing test" identifies and counts these fragments of DNA and is sensitive enough to detect the very small increase in amount of chromosome 21.

The new test correctly identified all the chromosomal abnormalities in a specially selected sample of 18 pregnant women - nine cases of Down's syndrome and two other inherited disorders or aneuploides caused by an abnormal number of chromosomes.

Dr Stephen Quake and his Stanford team say they need to repeat their study in a larger number of women, but they are confident that it could be used routinely in hospitals a few years from now.

Baby girl with Down's
Down's syndrome is caused by an extra chromosome

Dr Quake said: "Non-invasive testing will be much safer than current approaches."

One of the current ways of confirming the syndrome is amniocentesis, in which a needle is used to take a sample of the fluid within the womb.

Approximately one in 100 women who have the test will miscarry as a result, according to the Royal College of Obstetricians and Gynaecologists.

A similar invasive procedure that takes a tissue sample from inside the womb, called chorionic villus sampling, carries a miscarriage risk of two in 100. It can be carried out at an early stage of pregnancy than amniocentesis - after 10 weeks gestation compared with after 15 weeks for amniocentesis.

Dr Quake said the new DNA test could be carried out at an even earlier stage of pregnancy than the current tests, giving women more time to make choices about their pregnancy.

Dr Lyn Chitty, an expert in genetics and foetal medicine at University College London Hospital, said: "This is a potentially exciting development which may take us closer to a safer, non-invasive test for Down's syndrome and the other major aneuploidies, trisomies 13 and 18."

DOWN'S SYNDROME TESTS
Screening only identifies women at increased risk of having a baby with Down's based on the woman's age, proteins in her blood and a scan of the baby
Existing diagnostic tests - chorionic villus sampling and amniocentesis, which sample tissue or the fluid around the baby, respectively - are invasive, requiring a needle to be inserted into the womb
The new DNA blood test is diagnostic and non-invasive

She said other scientists were looking at different genetic markers in maternal blood for Down's. However, these tests would not work in all women, unlike the DNA test.

Carol Boys, chief executive of the Down's Syndrome Association, said: "There is no question that these non-invasive tests will be introduced in the next few years.

"It's therefore incredibly important that potential parents are given accurate information on Down's syndrome before they make a choice about whether to terminate or not.

"We don't consider Down's syndrome a reason for termination, but we recognise that bringing up a child with Down's syndrome isn't right for everyone.

"The more informed parents are, the better the position they are in to make the choice that is right for them."


SEE ALSO
Down's test 'at five weeks'
16 Jul 03 |  Health
Down's test miscarriage warning
17 Sep 08 |  Health
Down's syndrome test: Your views
07 Oct 08 |  Have Your Say

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