A more accurate prenatal test to predict Down syndrome

Screening looks at baby’s DNA in mother’s blood

Prenatal Test

PUT TO THE TEST  Prenatal blood tests that analyze fetal DNA are more reliable than standard screening methods for detecting whether babies carry certain extra chromosomes that can cause birth defects.

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A prenatal test that examines a baby’s DNA in a sample of the mother’s blood is much more accurate for detecting Down syndrome for most women than standard screening methods are.

In a study of 15,841 pregnant women, babies identified by the DNA test as having an extra copy of chromosome 21 had an 80.9 percent chance of actually having Down syndrome. In comparison, only 3.4 percent of babies that tested positive with standard screening methods had Down syndrome, an international team of researchers report online April 1 in the New England Journal of Medicine.

In the United States, one in 691 babies is born with Down syndrome. Standard screening methods include ultrasound and maternal blood tests that measure certain proteins. If these methods indicate a problem, doctors may perform more invasive tests, such as amniocentesis, which can raise the risk of miscarriage. More accurate screening methods could reduce the need for further testing.

In the study, the fetal DNA tests — in use since 2011 — identified nine false-positive cases of Down syndrome among 15,803 babies, for a false-positive rate of 0.06 percent. Standard screening methods found 854 false-positive cases, a rate of 5.4 percent. The DNA test was also more reliable for detecting extra copies of chromosomes 13 and 18, which can cause miscarriage, severe birth defects, stillbirth or death in infancy.

In about 3 percent of women, researchers could not get enough fetal DNA to conduct the test. Those women were more likely to be overweight and at higher risk of having babies with Down syndrome or conditions related to other trisomies. 

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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