"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.
Fetal DNA Tests: Perspective
The findings confirm what experts in the field have known, says Joe Leigh Simpson, MD, professor of obstetrics and gynecology at Florida International University in Miami. "Many different labs can in fact verify when a male pregnancy exists. This helps to confirm that the next steps down the path are taken on some firm ground," he says.
He reviewed the findings for WebMD but wasn't involved in the research. He reports being on the scientific advisory board of BioDx, involved in cell-free DNA research, and RareCells.
He agrees that the hope is to use the fetal DNA test to rule out or identify certain conditions early in the pregnancy. ''There are a certain number of conditions in which a female would not be affected [typically], hemophilia being an example," he says.
Simpson says the issue of using the tests for ''family balancing'' is often brought up in debates about the use of the tests. For instance, a family has two boys and is hoping for a girl and decides to take a gender test.
In his experience, the number of people who are going to terminate a pregnancy simply based on the finding that the fetus is the ''wrong'' sex is not an issue, although it is a concern voiced by some experts.
"It's a bogeyman everyone raises but I'm not sure it's valid in America," Simpson tells WebMD.
The test will not likely replace more invasive tests such as amniocentesis in the near future to detect such conditions as Down syndrome, says Mary Norton, MD, professor of obstetrics and gynecology and director of perinatal research at Stanford University School of Medicine. She also reviewed the research.
A test of fetal DNA in maternal blood to detect Down syndrome is under development.
That is because the test for Down syndrome has limitations. The most likely role for the test, at least for the near future, she says, would be ''to identify those women at high enough risk [of carrying a fetus with a birth defect] to go on to amnio."
However, by doing that, she says, it will spare many other women who are deemed not high risk the expense, stress, discomfort, and risk of amnio, she says.