Scientists say they have developed a blood test for women that can detect three times as many cases of Down's syndrome as existing tests, without risk of miscarriage and at far less cost.

Researchers said the test could be available for widespread use in two or three years, after further studies are done to confirm its effectiveness.The existing test uses a procedure called amniocentesis, in which a small amount of the amniotic fluid surrounding the fetus is withdrawn and fetal cells in the fluid are examined for the presence of chromosomal defects.

The procedure costs about $1,000 and leads to miscarriages in about one in 200 mothers who receive it, doctors said.

The new test, done by measuring the levels of three substances in the mother's bloodstream, identifies women at highest risk of having children with Down's syndrome, said one of the authors of the report, Jacob Canick, a biochemist at Brown University and Women and Infant's Hospital, both in Providence, R.I.