Maternal Blood Test Can Determine Sex of Fetus at 7 Weeks

TUESDAY Aug. 9, 2011 -- A simple blood test from mom can spot the sex of a fetus as early as seven weeks into a pregnancy, researchers report.

Because it is noninvasive, the test doesn't carry the same risks, such as miscarriage, as other methods for determining gender.

According to the U.S. research team, the test works by scouring free-floating DNA in the mother's blood, looking for "Y" or male chromosomes. If no Y chromosomes are detected, the fetus is presumed to be a girl.

Although the test does predict "boy" or "girl" with great accuracy, it was not created for this purpose but to detect certain medical conditions instead, the team note.

Still, "there is no reason why it couldn't be [used to determine fetal gender] - the technology is widely available and the test does not require specialized software or the like," said Dr. Diana W. Bianchi, senior author of a paper appearing in the Aug. 10 issue of the Journal of the American Medical Association.

But, she stressed that the screen was developed to spot certain sex-linked medical conditions early in gestation. "The major implications of this study are for three different groups of conditions, two of which are genetic conditions," Bianchi said.

The first of these would be X-linked or sex-linked conditions such as hemophilia, which usually affects boys.

If a woman knows, either because she has a brother or previous child with hemophilia, that she is at risk for having a baby with hemophilia, she would normally undergo invasive genetic testing, Bianchi explained.

That would mean either amniocentesis -- drawing amniotic fluid directly out of the womb -- or chorionic villus sampling, which involves taking tissue directly out of the placenta. Both carry a "small but real" risk of miscarriage, said Bianchi, who is executive director of the Mother Infant Research Institute at Tufts Medical Center in Boston.

The other condition is called congenital adrenal hyperplasia, a recessive condition that can result in girls having masculine-like genitalia and boys entering puberty early (as early as 2 or 3 years old). It is usually treated by giving the mother steroids.

And the third group involves fetuses with "ambiguous genitalia," meaning it's not clear from the fetal genitalia as seen on an ultrasound picture whether the baby is boy or a girl.

A blood test to detect this can help parents "begin to think about what should happen once the baby is born," Bianchi said.

In the study, she and her colleagues looked at 57 studies from the past 15 years involving about 6,500 pregnancies, roughly half producing boys and half producing girls.

The test correctly detected the gender of the baby virtually all of the time.

When the test was performed using maternal urine or before seven weeks' gestation, it was not accurate, the researchers said.

"A prior study out of Britain found that the test reduced by 50 percent the number of [pregnant] women who need an invasive procedure," Bianchi added.

The screen is already used routinely in some European countries but not in U.S. doctors' offices or hospitals. "The test is [also] available direct to consumers online -- but I have no information as to what techniques the online companies are using," Bianchi said.

She reported that she owns stock in and receives honoraria as a clinical advisory board member of Verinata Health Inc., a biotechnology company working on non-invasive genetic tests for pregnant women.

Given that the test is available direct-to-consumer and will likely become more widespread in the U.S., the blood screen could theoretically be used to test for gender alone, leading to some ethical issues. No one can predict, however, if parents will use the test to try to select the gender of their baby by aborting a fetus of the opposite gender.

"A lot of people would like to know more about the reasons that women or couples choose to terminate pregnancies but it's difficult research to do. It's a private topic and it's politically charged," said Susannah Baruch, a lawyer and policy consultant with Generations Ahead, a non-profit organization focused on ethical issues surrounding genetic testing, especially sex selection.

More information

There's more on prenatal testing at the March of Dimes.

Posted: August 2011


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