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 ...