But new research carried out at St. Bartholomew's Hospital, in London, shows that although the test diagnoses about 100 Down's babies a year, it also results in the miscarriage of 400 healthy fetuses.

The amniocentesis test involves a hollow needle being inserted into the womb to remove fluid which can then be tested for Down's and other conditions. The test has long been known to carry a risk of miscarriage, but this is the first time that the benefits and risks have been compared in this way.

Professor Rebecca Smith-Bindman, who carried out the research while on sabbatical from the University of California, in San Francisco, said she wants now wants to see the amniocentesis being carried out more selectively.

She told national newspapers: "These women were recommended for amniocentesis on the basis of ultrasound scans which relied on features that are mostly harmless to the baby and which almost all disappear during pregnancy."

The amniocentesis is carried out on pregnant women if a routine blood test or an ultrasound have raised any worries.

Miscarriage risk

The risks of having a child with Down's Syndrome rise with age. For a woman of 35 the risk can be around one in 300. This is becoming more of a problem as growing numbers of women are choosing to have children later in life. Every year about 1,000 babies in the UK are found to have Down's.

The condition, which is caused by a chromosome abnormality, often leads to learning disability and heart defects.

Leading health experts have called for the use of a non-invasive test in a bid to protect the unborn baby. Medical research charity, Action Research, is currently funding research into a non-invasive test.

Professor Nicholas Fisk, of the Imperial College School of Medicine at Queen Charlotte's Hospital, London, said the new tests would analyse special fetal cells found only in the mother's blood during early pregnancy. "The reality is that most women offered a chance of pre-natal diagnosis opt for it, which puts a fetus, with an abnormality or not, at risk.

"The success of this procedure may introduce pre-natal testing for genetic and chromosome abnormalities which will avoid the risk of miscarriages. This in turn may help assist with early treatments to reduce or prevent handicap in utero, or offer parents early counselling advice."