Early Down's test 'more sensitive'

A more accurate test for Down's syndrome which can also be given earlier in pregnancy than current checks has been developed, say experts

A study of 1,000 pregnancies found the test of foetal DNA in maternal blood can show a baby is "almost certainly" affected or unaffected by Down's.

The King's College London team behind it said it could help women decide if they needed further, invasive tests.

The Down's Syndrome Association said the new test was not "imminent".

Around 750 babies are born with Down's syndrome each year in the UK.

The condition is caused by the presence of an extra copy of chromosome 21, which occurs by chance.

'Nearly diagnostic'

Women are currently tested between weeks 11 and 13 of pregnancy. They have an ultrasound, during which a pocket of fluid at the back of a baby's neck - the nuchal translucency - is measured. Babies with Down's syndrome tend to have more fluid than normal.

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Testing for Down's syndrome

• All pregnant women are offered screening for Down's syndrome
• This combines an ultrasound scan of the baby and a blood test for the mother
• Experts then estimate a woman's chance of having a baby with Down's
• Those at higher risk can then opt to go for an invasive diagnostic test that will tell them if their baby has Down's
• This could be an amniocentesis or chorionic villus sampling
• Both carry a one in 100 risk of miscarriage
• The new blood test could reduce the number of high risk women needing an invasive test by adding an extra level of screening

Women also have a blood test to check for abnormal levels of certain proteins and hormones.

They are then given an estimation of the chances of their child having Down's - which also takes their age into account - such as one in 150 or one in 700.

Each of the three elements is only an indication.

But based on the result, those with a higher estimated risk can have one of two invasive and potentially risky tests.

Chorionic villus sampling (CVS) involves testing a small sample of the placenta, while an amniocentesis tests the amniotic fluid around the baby.

Both tests carry a one in 100 risk of miscarriage.

Prof Kypros Nicolaides, who is leading the research and also developed the nuchal fold test, says the foetal DNA (cfDNA) test is much more definitive.

The test shows there is either more than a 99% chance, or less than one in 10,000 that their baby has Down's syndrome.

The research showing it is more sensitive, and less likely to offer a false-positive result, is published in Ultrasound in Obstetrics and Gynaecology.

Prof Nicolaides said: "This test is nearly diagnostic. It tells you almost certainly your baby has Down's or almost certainly it does not.

"From a woman's perspective, that is a much more clear message about what to do next."

Between 3% and 5% of pregnant women currently undergo invasive testing.

The foetal DNA test has seen a rate of less than 0.5%.