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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 Live Science 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.
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 Live Science.
Most pregnant women in the United States get an ultrasound between 18 and 22 weeks of pregnancy that looks for fetal anomalies.
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. 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.
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.