Blood Test Predicts Baby's Sex at 7 Weeks

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Some prenatal gender tests that use mom's blood are very accurate at determining baby's sex, a new study finds. But curious parents-to-be should be wary of online marketers that claim to be able to figure out fetal gender using just a woman's urine.

New research to be published Aug. 10 in the Journal of the American Medical Association finds that after seven weeks into a pregnancy, tests that analyze mom's blood for fetal DNA can correctly identify a male fetus 95.4 percent of the time and a female fetus 98.6 percent of the time on average. In comparison, tests that analyzed DNA from urine instead of blood were only accurate 41 percent of the time, said study researcher Diana Bianchi, a reproductive geneticist at Tufts University School of Medicine.

"It was worse than flipping a coin," Bianchi told LiveScience.

Why baby's sex matters

Ultrasound imaging can sometimes reveal the sex of a baby as early as 11 weeks into pregnancy, though the results are wrong as much as 40 percent of the time. Most pregnant women in the United States get an ultrasound between 18 and 22 weeks of pregnancy that looks for fetal anomalies. At that point, the fetus' sex can be determined with high accuracy. [5 Myths About Women's Bodies]

Some people don't like to wait that long. Chelsea Gladden, who blogs at breezymama.com, told LiveScience that she and her "swollen ankles" needed the excitement of finding out her baby's sex about halfway through the pregnancy, but said she would have found out earlier if she could have.

"I was definitely consumed with finding out," Gladden said.

But curiosity isn't the only reason for earlier gender testing. Certain genetic disorders are linked to the X chromosome, so they overwhelmingly affect males, whose XY sex chromosomes mean they lack the "backup" X that women have. Families at risk for these disorders can now opt to have amniocentesis, in which the fluid that cushions the fetus in the womb is extracted and tested, or a procedure called chorionic villus sampling, both of which carry a small risk of miscarriage.

A non-invasive blood test would cut down on such testing by 50 percent because moms carrying female babies wouldn't need to worry, said Bianchi. Bianchi is on the advisory board and holds stock options in the biotechnology company Verinata Health, Inc., which has the goal of developing non-invasive fetal abnormality tests.

Another disorder, congenital adrenal hyperplasia, or CAH, disrupts hormone balance, resulting in a female fetus taking on masculine traits. Moms carrying fetuses with CAH take steroids during their pregnancies, which can have unpleasant side effects. If fetal sex were known earlier, Bianchi said, the moms carrying male fetuses with CAH would be able to skip the steroids.

Chromosome-based diagnoses of gender are also important to parents whose children are born with ambiguous genitalia. If an ultrasound reveals genitals that could be male or female, Bianchi said, knowing the baby is XX or XY can give parents a road map for what gender to raise the child. [The Truth About Genderless Babies]

It's a boy! (or girl)

Blood tests for fetal gender aren't available clinically in the United States, Bianchi said, though they are used in Europe for diagnosis in high-risk pregnancies. A number of companies do sell blood and urine tests of fetal sex to parents online for several hundred dollars.

Bianchi and her colleagues combed through the scientific literature to find studies on those blood and urine tests that use fetal DNA from mom's blood to identify sex. Only males have Y chromosomes, so if Y chromosomes are found in mom's blood, she's likely carrying a baby boy. If no Y chromosomes are found, she's probably expecting a daughter.

After excluding studies that lacked data or were too small, Bianchi and her team came up with 57 studies of prenatal sex tests to analyze. They found that urine tests were extremely unreliable, possibly because by the time fetal DNA is filtered from the blood into the urine, it's been broken down.

Blood tests, on the other hand, revealed fetal DNA quite early. Before seven weeks, blood tests correctly identified male fetuses only 74.5 percent of the time. After seven weeks, however, accuracy went up. Tests conducted between seven and 20 weeks of pregnancy accurately identified baby boys about 95 percent of the time and baby girls about 99 percent of the time. After 20 weeks, these test were extremely accurate, pegging boys as boys 99 percent of the time and girls as girls 99.6 percent of the time.

Parents of at-risk pregnancies should talk with their doctors, Bianchi said, because blood tests could help prevent more invasive procedures down the road. Bianchi said she sends samples to the U.K. when she really needs a test done.

But for moms and dads who just want to know what color to paint the nursery, Bianchi recommends patience.

"Currently, they would have to go to the Internet, and I would say that they should be wary," she said. "There is not a whole lot of transparency in those methods or the actual performance results."

The study was funded by the National Human Genome Research Institute.

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.