They say all pregnant women should be offered the test, regardless of their age and view the new screening method as practical, acceptable and cost-effective.
Professor Nick Wald and colleagues at St Bartholomew's Hospital in London, say in tomorrow's issue of the British Medical Journal that they have proved the effectiveness of the screening in a three-year study involving 12,000 women and found it to be better than current methods of screening for Down's syndrome, which are crudely based on a woman's age.
The new test is used to predict risk only and is quite harmless and routine. Those women found to be at high risk are offered amniocentisis, which will show if the foetus has Down's syndrome.
Amniocentisis, in which a sample of amniotic fluid is taken for analysis, carries a very small risk of spontaneous abortion, up to 1 per cent. The new risk assessment means that women who are not at risk can avoid amniocentisis.
The different way of screening for Down's syndrome is almost twice as effective. Using the new method, called the Bart's Triple Test, 12 out of 25 Down's pregnancies were detected, or 48 per cent.
With the traditional method of offering amniocentesis to older women, a maximum of only seven Down's babies would have been detected, the doctors say.
The blood test, developed by researchers at St Bartholomew's four years ago, involves looking at 16 weeks for abnormal levels of three substances - alpha-fetaprotein, which is produced by the foetus and two hormones, oestriol and human chorionic gonadotraphin.
Taking the results and the woman's age into account, a computer program is used to predict her risk of carrying a Down's baby. If the risk is greater than one in 250, whatever her age, amniocentisis is offered. This method of screening is now offered routinely to women in the St Bartholomew's catchment area and in some other parts of the country.
Anne Kennard, ante-natal screening co-ordinator in the Department of Environmental Preventive Medicine at St Bartholomew's, said doctors hoped that other areas would now begin to offer the 'triple test'.
About 800 to 1,000 Down's babies are born every year, three- quarters of them to women under the age of 37.
However, the risk of having a Down's baby is many times higher in older women and the risk rises dramatically with age. For a woman under 20, the risk is about one in 2,000; at 45, it is about one 20; and at 48 about one in 16. For this reason health authorities offer amniocentisis routinely only to women who have reached their middle or late 30s.
The researchers have costed their screening method and estimate that it costs pounds 38,000 to avoid one Down's baby. The lifetime costs of caring for one Down's child have been estimated at pounds 120,000.
But the doctors say in the Journal: 'The most important reason for screening, however, is not financial: it is the avoidance of handicap and of distress to the families concerned.'Reuse content