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.
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.
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%.
Next month, the professor and his team are to begin a two-year prospective study of 20,000 women in NHS hospitals to further assess the test.
However it currently costs around £400, so Prof Nicolaides says – if the cost does not fall – it may be that the NHS could use the conventional test (which costs £180) for all pregnant women, then the foetal DNA test for those at a higher risk – perhaps 10-15% of all pregnancies.
He said his aim was to offer women clearer information to allow them to make choices about how they should proceed.
“It has been trendy to say we must involve patients in the decision-making process, but it has often been something we only pay lip-service to.
“If the risk is say one in 250, how do they decide? When they have much more clarity, a clearer result, it is made easier.”
Carol Boys, chief executive of the Down’s Syndrome Association (DSA) said: “The latest results from Prof Nicolaides and his team at King’s College show that the use of an early non-invasive blood test that could be used throughout the national screening programme is still a fair way off.
“The DSA considers it far more important at this point to focus on providing relevant, accurate and up-to-date information about Down’s syndrome, delivered by midwives and associated health professionals, who have received our targeted training prior to any screening test.
“We are currently seeking full funding to ensure that our Tell it Right, Start it Right training can be rolled out nationally in readiness for the time when the non-invasive diagnostic test in early pregnancy is a reality in the UK.
“We do not believe that this is imminent.”