They say such test is one of the "holy grails of medicine".

At the moment tests for conditions such as Down's syndrome have a two per cent risk of miscarriage.

In order to detect chromosome and genetic abnormalities in the unborn child, doctors must use a procedure like amniocentesis, which involves placing a needle in the womb.

But scientists at Imperial College London believe they are close to perfecting a technique that can detect such defects using a sample of the mother's blood.

Finding the baby's cells

Dr Irene Roberts leads the research team. Their approach involves isolating foetal cells - known as haemopoietic stem cells - in the mother's blood.

There are very few cells from the unborn baby in the mother's blood stream - five for every 20,000 million maternal red blood cells.

Dr Roberts' team has isolated the haemopoietic stem cells, which can renew themselves when exposed to certain processes.

The team hopes to increase the number of foetal cells to 100-200 by using growth factors.

Conference presentation

Dr Roberts presented her team's work to the Pregnancy into the Next Millennium conference in London.

She said: "Development of a non-invasive test for prenatal diagnosis with equivalent accuracy to current invasive procedures but without any risk of fetal loss is one of the holy grails of fetal medicine."

She said the team had almost completed developing a method of distinguishing between mother's and baby's cells.

"Our final task is to apply all our information to see whether our predictions are correct and form the basis for a reliable non-invasive test," she said.

"These potentially exciting experiments are now under way."

A spokesman for the National Childbirth Trust, which campaigns on pregnancy and childbirth issues, said: "The prospect of this test gets a massive thumbs up from us and is something we would welcome with open arms.