A simple test using pregnant women's blood scored well in determining the sex of unborn babies as early as the seventh weeks of gestation, U.S. researchers say. Stephanie A. Devaney of the National Institutes of Health in Bethesda, Md., and colleagues says the non-invasive prenatal test using cell-free fetal DNA provides an alternative to invasive techniques for some heritable disorders. The researchers conducted a meta-analysis of about 57 studies involving 80 data sets -- 3,524 male-bearing pregnancies and 3,017 female-bearing pregnancies. Overall performance of the test to detect Y chromosome sequences was 95.4 percent sensitive, specificity was 98.6 percent, positive predictive value was 98.8 percent and negative predictive value was 94.8 percent. Although maternal blood tests after seven weeks were reliable, the best performance was after 20 weeks' gestation. Tests before seven weeks were unreliable, as were all tests using urine, the study finds. The findings are published in the Journal of the American Medical Association.