Boy or Girl? Fetal DNA Tests Often Spot On

Aug. 9, 2011 -- Using fetal DNA from a mother's blood to determine the unborn baby's sex is highly effective, according to a new review of the research.

"After seven weeks of gestation, the accuracy of fetal sex detection is very good using maternal blood," says researcher Diana W. Bianchi, MD, a reproductive geneticist and executive director of the Mother-Infant Research Institute at Tufts Medical Center in Boston.

At seven weeks, she found 95% accuracy. "After 20 weeks, it was nearly perfect."

The research involves tests done by hospital or research groups worldwide. "It shouldn't be confused with what the direct-to-consumer companies are advertising [to help parents find out the gender of their unborn infants]," she says.

The hope, Bianchi says, is to use these research-based tests to detect unborn babies at risk for sex-linked disorders, such as hemophilia, and other genetic disorders early in the pregnancy. Depending on those results, it could delay or rule out the need for more invasive tests such as amniocentesis.

The review is published in the Journal of the American Medical Association.

Fetal DNA Tests: Evaluating the Research

Bianchi and her colleagues evaluated 57 published studies. They pooled the data and results from these studies. In all, the research represented 3,524 pregnancies bearing boys and 3,017 pregnancies bearing girls.

The studies looked at tests of fetal DNA in maternal blood and urine. For about a decade, experts have known that maternal blood contains fetal DNA.

Overall, the tests using blood detected the Y chromosome indicating a boy 95.4% of the time. The studies that evaluated DNA in maternal urine were unreliable, the researchers say.

As the pregnancy went on, the blood test became more reliable. That is believed to be due to the increased concentration of fetal DNA within the mother's blood, Bianchi says.

By 20 weeks, the test was 99% accurate, she says.

The test is not routinely done by doctors in the U.S., Bianchi says. That's partly due to the lack of reimbursement by insurers, she says. The research she reviewed came from hospitals and research groups worldwide.

Fetal DNA Tests: The Future

Eventually, Bianchi says, the blood tests may help detect, early in pregnancy, fetuses with certain medical conditions that affect one sex more than the other.

For instance, she says, a condition called congenital adrenal hyperplasia can make girls' genitals appear masculine-like. If a pregnant woman were at risk of delivering a girl with this condition, she could be treated with steroids to prevent those effects.

If the Y chromosome were found on the test, the woman would be spared the treatment.

For consumers who are curious about the sex of their unborn baby, Bianchi says her research cannot vouch for the effectiveness of those tests.

Bianchi reports serving as a member of the scientific and clinical advisory board of Verinata Health Inc., a biotech company. She also holds stock options and receives honoraria from Verinata. The company had no role in the current research. It was funded by the National Human Genome Research Institute.

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