The test is being offered to parents in Australia who are undergoing IVF because they want their child to avoid disorders such as cystic fibrosis.

It would be a tragedy if, after these parents got the all-clear from the pre-implantation genetic test, they found out their child had Down�s syndrome

Mandy Katz

Parents who opt for pre-implantation screening do not have fertility problems, but during IVF, doctors have access to fertilised embryos outside the body and can take the opportunity to run tests on them.

The defective gene involved is passed to some offspring, but not others, and doctors screen the embryos to select and implant only those which do not carry them.

Risk of age

Many of these have already had a child with the disorder, and, as a result, are either in their 40s, when they opt for screening.

The mother's age substantially increases the chance that a child might also have Down's syndrome - but the new test can rule this out, and help doctors select embryos with no sign of either defect.

It avoids the need for the mother to have a separate test, amniocentesis, which in a small number of cases, can cause a spontaneous miscarriage.

It was developed at the Monash Institute of Reproduction and Development by Mandy Katz, a PhD student.

She said that prior to her test, an embryo could be declared free of a single gene defect, yet still be carrying Down's.

She said: "It would be a tragedy if, after these parents got the all-clear from the pre-implantation genetic test, they found out their child had Down's syndrome."

Down's, which occurs in one in every 700 live births, is caused by an extra bundle of genetic information, called chromosome 21.

Children born with the defect have differing degrees of developmental problems, and distinctive facial features. They are also more likely to have congenital heart defects which require surgery.

Ms Katz said that the DNA fingerprinting technique was more cost-effective and only needed a single cell to make both diagnoses.

The accuracy of the Down's test was as much as 94%, she said.

The research team is now hoping to extend the combined test to cover other single-gene defects, so that more parents could be helped.