Blood test early in pregnancy to detect baby’s gender

By Dr. Ananya Mandal, MDAug 12 2011

While carrying the child, a mother's blood has cell-free fetal DNA floating around in it, and it's possible to use this to test for the baby's sex. In a meta-analysis that covered more, the test was found to be 95% efficient in guessing the sex of the fetus — and it works from as early as seven weeks. As the fetus and the placenta develop, the amount of DNA present in the mother increases, boosting the accuracy of the test, which hits its peak at 20 weeks.

This method is apparently already widespread in Europe, but America hasn't adopted the practice yet — despite the fact that it could mean a faster, easier way to find out the sex of the baby Stephanie Devaney of the National Institutes of Health in Bethesda, Md. and colleagues reported in the August 10 issue of the Journal of the American Medical Association.

In order to look at overall performance of the blood tests, the researchers conducted a review and meta-analysis of 57 studies conducted between Jan. 1, 1997 and April 17, 2011, totaling 80 data sets on 3,524 male-bearing and 3,017 female-bearing pregnancies. Generally, they found high sensitivity and specificity for Y-chromosome detection in maternal blood (95 percent and 99 percent, respectively).

Devaney and colleagues noted that a disadvantage of fetal DNA blood testing is the need to validate female sex, because the test looks for male, or Y-chromosome, DNA. Also, the test is not currently available at the doctor's office, has not yet been approved by the Clinical Laboratory Improvement Amendments (CLIA), and is not currently reimbursed by insurers. They also noted that many of the included studies were small, so it would be “beneficial to help validate test performance under highly controlled testing conditions.”

This could also help test manufacturers to “ensure that their claims are accurate,” as some companies that directly market the tests to consumers claim their products have an accuracy of 95 percent to 99 percent as early as five to seven weeks' gestation.

The real question is whether there's enough of the fetal DNA in the mother's system to test for chromosomal abnormalities, which would allow for other kinds of genetic testing that are arguably more important to the future baby's health.