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:
Doctors in Europe regularly use the test if a baby might be at risk for a sex-linked disease. For instance, if a mother carries the gene for hemophilia, a disease that affects males, a son might have the disease but a daughter would be a healthy carrier, like her mother. If the fetus is male, further genetic tests might be necessary to determine if the baby will have hemophilia; if it’s female, they aren’t.
The test hasn’t caught on in the same way in the US. Parents-to-be can get the test only by ordering it directly from a manufacturer, not from their doctor. (Some versions of the test available in the US are less accurate than those analyzed in the current study.)
The upside of the fetal DNA method is that it can reveal gender earlier and with more accuracy than an ultrasound, and without the invasiveness — and thus small risk of miscarriage — of amniocentesis and chorionic villus sampling.
The downside, some warn, is easier gender selection. In parts of Asia, parents’ preference for boys over girls has led to a gender imbalance. Some bioethicists and doctors fear that being able to know a baby’s sex so early in pregnancy would lead to more sex-selective abortions in the US.
Similar fetal DNA tests are also being used to detect genetic diseases, such as Down’s syndrome.
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