Researchers seeking a safer way to detect Down's syndrome in a fetus say combining blood and ultrasound tests from the first and second trimesters would be more accurate than standard screenings and reduce the need for riskier testing.
The method would spare many women the need for a definitive test called amniocentesis, in which a needle withdraws fluid from the amniotic sac.
That triggers miscarriage about 1 percent of the time.
The research is reported in today's edition of the New England Journal of Medicine.
In an editorial in the journal, Dr. Joshua Copel and Dr. Ray Bahado-Singh of Yale University argue that the method won't gain wide acceptance because it requires withholding results of first-trimester tests until they are combined with the later data. Women might prefer an earlier warning so they could terminate the pregnancy safely, they said.
However, the lead researcher, Dr. Nicholas Wald, said that compared with the testing that is standard in the United States, the new system would prevent the miscarriages of about 1,400 healthy fetuses each year and detect about 800 more fetuses with the chromosomal abnormality.
"You've got an improvement in detection, but the real gain is the reduction in the number of women who have to have an amnio," said Wald, chairman of the Wolfson Institute of Preventive Medicine in London.
The screening also could reassure expectant parents their baby doesn't have Down's syndrome, a condition in which an extra chromosome causes mental retardation and physical abnormalities.
About one in 700 babies has Down's syndrome, which is marked by a broad, flat face with slanting eyes _ and an early death.
Older women have a higher risk of delivering a baby with Down's syndrome _ for example, for a woman who is 40, the rate is one in 106.
Wald and colleagues developed a complicated computer program that integrates results of tests in both trimesters. First, a woman is given an ultrasound and a blood test for a protein linked to Down's syndrome, both done 10 to 14 weeks into the pregnancy. A month later, she has blood tests for abnormal levels of four substances associated with a high risk of Down's syndrome.
Wald's computer program correctly identified 85 percent of Down's syndrome cases. Standard screening detects 46 percent.