A test that can detect Down’s syndrome from the blood of pregnant women has raised the prospect of routine screening for the condition for every expectant mother who wants it.

Genetic markers that show whether a foetus has the chromosomal disorder can now be identified in the mother’s bloodstream, after research that promises the first reliable noninvasive prenatal test.

The experimental procedure, developed in Hong Kong, has been shown to diagnose 90 per cent of Down’s syndrome cases in a small trial, while also correctly identifying 97 per cent of foetuses that do not have the condition.

If its accuracy can be improved and it is validated in larger patient trials, which scientists believe should take three to five years, it would transform prenatal testing for Down’s.

At present this is provided only for women at high risk of having a Down’s baby because the current procedure is invasive and can cause a miscarriage. It requires amniocentesis or chorionic villus sampling (CVS), which involve inserting a needle into the womb to remove amniotic fluid surrounding the foetus, or a small piece of the placenta. Both techniques provoke a miscarriage in one in 100 women, and 320 healthy pregnancies are lost each year in Britain because of Down’s tests. These could be prevented by the noninvasive prenatal diagnosis (NIPD) test, which could eventually be part of standard antenatal screening.

The advance, however, will also create ethical dilemmas for many couples following positive tests. There are fears that the simpler procedure and more extensive screening could lead to more abortions.

Nigel Carter, of the Wellcome Trust Sanger Institute, near Cambridge, whose team is working on the technique, said: “As well as your ultra-sound scan, you would have a few millilitres of blood taken for your Down’s test. It’s the sort of procedure that could move from being offered only to mothers at risk, to become a more normal screening procedure.”

Lyn Chitty, of Great Ormond Street Hospital for Children, London, said: “It isn’t offered for Down’s on a clinical basis at the moment, but if you talk to people in the field the majority think we will be there within five years. This whole technology really is going to change prenatal diagnosis.”

A reliable blood test would enable many more women to learn whether they are carrying a foetus with Down’s. Those who then choose an abortion would be able to have it earlier, while those who do not would have more time to prepare themselves for bringing up a child with learning disabilities.

At present, women who would not consider a termination rarely have amniocentesis or CVS, as there is little point in endangering a pregnancy just for the sake of an early warning.

Down’s syndrome occurs when three copies of chromosome 21 are inherited instead of the usual two, causing learning difficulties, often accompanied by serious cardiac defects and a high risk of early-onset dementia. It affects between 1 in 800 and 1 in 1,000 pregnancies, and the risk rises with maternal age, especially after 40.

Despite the age link, most Down’s babies are born to young women, as these simply account for more births.

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