Mom’s blood can reveal baby’s sex by 7 weeks of gestation, study finds

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A boy is a boy is probably a boy. That’s according to a new study that finds that those noninvasive genetic tests used to determine whether a fetus is male or female are surprisingly accurate, as early as seven weeks of pregnancy.

The paper, released Tuesday in the Journal of the American Medical Assn., described a meta-analysis of 57 studies representing 3,524 male births and 3,017 female births.

The researchers found that tests that look for fragments of the baby’s DNA in a sample of the mother’s blood are about 95% to 99% accurate, depending on several factors. They can be used well before ultrasound (at 11 weeks) and aren’t invasive, unlike amniocentesis, which carries a small but real risk of miscarriage.


Here’s the general idea: A mother’s blood carries free-floating fragments of her child’s DNA during the time that she carries the fetus. Researchers can check for the child’s sex by looking for the “Y” chromosome, which only appears in males. If their sample turns up a “Y” chromosome, it’s a boy. If it doesn’t turn up a “Y” chromosome, it’s a girl -- although there’s the slight possibility that the test simply didn’t detect any “Y” chromosome in that particular maternal blood sample.

On the whole, the researchers found the tests were accurate when there was enough cell-free fetal DNA in the mother’s blood -- at seven weeks’ gestation or later. The best results were found after 20 weeks of gestation. Blood tests before seven weeks and any tests using urine were not reliable, the study found.

The idea that these tests might be so dependable could lead more families to use them to check the gender of their child or to look for sex-chromosome-based genetic conditions.

But expectant buyers beware, says science writer Karen Kaplan: There’s a whole industry of questionable genetic tests put out by “entrepreneurs” that promise to tell parents-to-be practically anything about their future children, from ethnic heritage to most viable future sports activities.

Follow me on Twitter @LAT_aminakhan.