For as long as it has existed, the prenatal test for Down's syndrome has stirred ethical controversy. As its main purpose is to allow women to decide whether to proceed with affected pregnancies, it has become a battleground in the abortion debate.

Down's screening, however, is contentious for another reason: it endangers pregnancies. It relies on amniocentesis and chorionic villus sampling, invasive procedures in which a needle is inserted into the womb. These provoke a miscarriage in one in 100 women, and cause the loss of 300 healthy pregnancies each year. As a result, such tests are offered only to the one in 20 women who is at high risk.

About 750 babies with Down's syndrome are born in UK ever year, 60 per cent diagnosed before birth.

This week, American scientists announced promising results for a non-invasive blood test for pregnant women, which can detect Down's without risk to the foetus. Other groups are pursuing similar research, and while nothing is yet ready for clinical use, it is probable that a test will be available soon.

On one level, this is good news. A safe replacement for the needle would prevent the loss of many healthy pregnancies. Another benefit is that testing could be done earlier than amniocentesis, which is performed between 15 and 20 weeks, allowing couples more time to make up their minds.

A non-invasive test could be extended to all prospective mothers. This would give peace of mind to hundreds of thousands, while detecting hundreds of cases that are currently missed. But there is little doubt that it would also lead to more abortions.

Some people regard this prospect with horror, including many parents of Down's children. While Down's causes learning difficulties, as well as heart defects and other health problems, they point out that most people with the condition lead happy and fulfilling lives. A routine test that leads to routine abortions, they say, would mean routine tragedy.

This ethical argument, though, is not as new as it seems. Society has decided that Down's is a sufficiently serious disability that terminations are legal even beyond the “social” limit of 24 weeks. If prenatal tests at 20 weeks are permitted, there are no logical grounds why earlier and safer procedures should not be allowed too.

There is also a strong compassionate case for the new test, which could even benefit children who are born with the condition. As today's screening options are so hazardous, they are not recommended even to high risk couples who would not consider an abortion. There is little point risking a miscarriage when the intention is to have the baby regardless.

Yet were the test entirely safe, that would not apply. Parents who would not terminate would learn early on in pregnancy that they were expecting a baby with the disorder. It would give them valuable time to come to terms and prepare, to the family's ultimate good.

If this test does not pose special ethical challenges, however, it still has the potential to cause distress if its eventual introduction is not properly managed.

At present, only about 32,000 high-risk women are offered tests that diagnose Down's reliably, after full counselling about the implications. When the new procedure comes in, 20 times this number will be suitable for such accurate diagnosis. There is a real danger that without detailed planning, not all of them will get the counselling they need to make informed decisions about how much they want to know, and how they might deal with unwelcome news.

The success of a Down's blood test will require more than clever science. Just as important will be careful thought about how it will be delivered by health professionals, and explained to pregnant women and their partners.

Mark Henderson is the science editor of The Times

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