A new blood test for pregnant women could nearly eliminate the use of a risky, invasive procedure for diagnosing Down syndrome.

New Zealand's incidence of Down syndrome, which can cause a range of intellectual and physical difficulties, rose by more than half from 1996 to 2002, a year in which 80 babies with the chromosomal disorder were born.

A woman's chance of having a Down baby rises steeply from the age of 35.

At present women are screened for the risk of spina bifida and chromosomal disorders based on their age, blood tests for certain proteins and hormones, and ultrasound scanning.

Those at increased risk are offered an invasive test: either chorionic villus sampling (a sample taken from the placenta) before 14 weeks into the pregnancy, or, after 14 to 15 weeks, amniocentesis (a sample of the fluid surrounding the fetus).

Both involve the insertion of a needle. For every 200 of these tests performed, one or two pregnancies will miscarry.

New Zealand has tried to reduce unnecessary use of these tests by improving the screening system last year, making more use of the existing blood tests. Five per cent of pregnant women were having amniocentesis before the changes.

It is estimated that up to 10 per cent of women who know their fetus has Down syndrome choose not to have an abortion.

Now a research group from the Chinese University of Hong Kong have developed a new form of blood plasma testing for Down syndrome based not on proteins or hormones, but on fetal DNA fragments in the mother's bloodstream.

Their idea was to determine the proportion of DNA molecules from chromosome 21, on the hypothesis that women with a Down fetus would have a higher proportion. This is because Down syndrome is caused by an extra copy of chromosome 21.

Reporting their study in the British Medical Journal, the researchers say their tests proved effective.

They used two types of DNA testing for 753 pregnant women from Hong Kong, Britain and the Netherlands at high risk of carrying a Down fetus and compared their findings with the results of diagnostic tests.

The better of their DNA sequencing tests produced a true-positive rate of 100 per cent and a true-negative rate of 97.9 per cent.

They say their testing could be offered to all women being considered for the invasive tests.

It also has the potential to become the first-line screening test and could be developed to screen for other chromosomal disorders.

Peter Stone, the professor of maternal fetal medicine at Auckland University, said yesterday that the new tests looked promising.

The technology would be expensive, but might prove cost-effective as an additional screening method for high-risk women and would reduce the amount of invasive testing, he said.

The National Screening Unit's manager of antenatal and newborn screening, Jane McEntee, said the new tests appeared to have potential benefits, but were subject to further clinical investigation.

It was too soon to consider introducing them to New Zealand.