Scientists Estimate Zika-Related Microcephaly Risk

This estimate is the result of an analysis of Zika virus infection rate and reports of microcephaly cases among infants born between summer 2015 and February of this year. The CDC-led team reported a 1 percent to 13 percent risk of microcephaly in fetuses whose mothers were infected during their first trimester (a range reflecting uncertainty about the accuracy of both Zika and microcephaly diagnoses). Infection during the second and third trimesters showed a “negligible association” with microcephaly, the researchers wrote in their report.

The authors concluded that this association could yet become apparent in other areas where Zika is being transmitted, and argue for “precautions to avoid [Zika] infection during pregnancy” and for “health care systems to prepare for an increased burden of adverse pregnancy outcomes in the coming years.”

Christina Chambers of the Center for Promotion of Maternal Health and Infant Development at the University of California, San Diego, told STAT News that although the analysis was “thoughtful,” the estimates of microcephaly risk were based on some assumptions—such as that all women are equally susceptible to Zika infection—that are yet to be tested.

“Clearly infection in any trimester seems to be associated with microcephaly so as they conclude, should be avoided,” Chambers wrote in an email to STAT. “It might make sense that first trimester carries the greatest risk but [that] remains to be seen as we get more individual level data.”