Scientists Confirm: Blood Test Can Tell A Fetus's Sex at Seven Weeks

The test can distinguish the sex of a fetus up to three months earlier than an ultrasound can.

By Valerie Ross
Aug 10, 2011 9:07 PMMay 12, 2023 4:23 PM
Woman carrying her baby girl at home


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What’s the News: 

A blood test can reliably tell a mother-to-be whether to expect a boy or girl as early as seven weeks into pregnancy, according to a new analysis published today in the Journal of the American Medical Association. The test can distinguish the sex of a fetus up to three months earlier than an ultrasound can, and doesn’t carry the slight risk of miscarriage that accompanies invasive tests such as amniocentesis.

How the Heck:

  • The test works by detecting tiny bits of fetal DNA floating through an expectant mom’s bloodstream. In particular, the test looks for little fragments of a Y chromosome, which  only males have. Some Y chromosome DNA in the blood sample means it’s a boy; none means it’s a girl.

  • Of course, this method isn’t perfect. The test could fail to recognize a minute amount of male DNA in a sample, or mistakenly detect a bit of a Y chromosome where there isn’t.

  • So, the researchers set out to determine just how accurate this test was. They analyzed all the data from 57 previous studies of the technique, looking at a combined total of more than 6,500 pregnancies.

  • At 7 to 12 weeks, the researchers found, the test determined the sex of a fetus with 95% accuracy. At 20 weeks, the test got it right 99% of the time.

What’s the Context:

Reference: Stephanie A. Devaney, Glenn E. Palomaki, Joan A. Scott, & Diana W. Bianchi. “Noninvasive Fetal Sex Determination Using Cell-Free Fetal DNA.” Journal of the American Medical Association, August 10, 2011. DOI: 10.1001/jama.2011.1114

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