Sufficient evidence has accumulated to infer a causal relationship between prenatal Zika virus infection and microcephaly and other severe brain anomalies, according to a Centers for Disease Control and Prevention study published today by the New England Journal of Medicine. The study evaluated the evidence to date using standard criteria used to identify potentially harmful exposures during pregnancy and evidence of causation. The timing of Zika virus infection associated with severe microcephaly and other severe birth defects of the brain appears to be in the late first or early second trimester, the authors said, noting that more research is needed to understand the full spectrum of defects caused by congenital infection and the relative risks for infants born to women infected at different times during pregnancy. At a press briefing on the study, CDC Director Tom Frieden, M.D., said, “Never before in history has there been a situation where a bite from a mosquito could result in a devastating malformation.” The study’s findings do not change CDC’s current Zika guidance for health professionals and the public, said lead author Sonja Rasmussen, M.D., director of the CDC’s Division of Public Health Information and Dissemination. For more information, visit and