Image caption The blood test looks for foetal DNA

A DNA blood test for Down's syndrome could save nearly all pregnant women from invasive tests like amniocentesis, say experts.

Invasive testing takes place in 3% to 5% of pregnant women in the UK - some 30,000 women - and increases the risk of miscarriage.

The new DNA blood test could bring this down to 0.1%, according to a study in the British Medical Journal.

Around one woman in every 100 who has an invasive test will miscarry.

Some faced with the dilemma choose not to go for a diagnostic test - which involves having a needle inserted into their bump to draw off a sample of placenta cells or some of the fluid that bathes the baby - particularly if their estimated risk of having a child with Down's syndrome is smaller than the chance of miscarriage.

This extra screen is non-invasive and would save many from needing further investigation Professor Kypros Nicolaides , One of the study authors

The non-invasive DNA blood test could offer another option.

How it works

Babies with Down's syndrome have an extra copy of chromosome 21, causing physical and intellectual impairments.

As DNA can cross the placenta from the baby to the mother, the blood test can look for this extra chromosome.

Scientists believe it should be rolled out as a screening test in the future based on their findings.

The latest study - the largest to date, based on 753 pregnant women in Hong Kong, the UK and the Netherlands - shows that it could bring the number of invasive tests down significantly, by about 98%.

Testing for Down's syndrome All pregnant women are offered screening for Down's syndrome

Screening entails an ultrasound scan of the baby and a normal blood test for the mum

From the results, experts can estimate a woman's risk of having a baby with Down's

Women at higher risk can then opt to go for an invasive diagnostic test that will tell them if their baby has Down's

There are two invasive diagnostic tests - amniocentesis and chorionic villus sampling

Both involve inserting a needle into the womb to collect cells or fluid from around the baby

The new blood test could reduce the number of high risk women needing an invasive test by adding an extra level of screening

This is because current NHS screening has a "false positive" rate of about 5%, meaning 5% will be told they are carrying a baby with Down's when they are not.

If these women were given the DNA blood test instead, almost all invasive procedures could be avoided, according to the researchers.

For example, each year in the UK some 30,000 pregnancy women undergo invasive testing and around 10% of these end up with a diagnosis of Down's.

Doing a DNA blood test beforehand would mean fewer than 4,000 women would still need an invasive test.

Professor Kypros Nicolaides of King's College London, who led the research along with colleagues from The Chinese University of Hong Kong, said the test would be welcomed by many women.

"Some women, understandably, are fearful of invasive tests.

"This extra screen is non-invasive and would save many from needing further investigation.

"Our study shows it is feasible to use in clinical practice."

He said the test was still too expensive and needed further study before it could be rolled out to be used routinely - something that could take 10 years.

The ultimate goal is to make it 100% accurate so that invasive tests could be dispensed with completely.

Currently, three women in every hundred that test positive would not actually have a baby with Down's.

Professor Lyn Chitty of University College London has also been trialling Down's DNA blood tests.

She said: "I suspect there are many women who would welcome such a test and it may lead to a lot more women accepting the offer for screening."

But she said it would be important to counsel women about the shortcomings of test.

"As yet, the results are not accurate enough to inform important decisions, like whether to continue the pregnancy."