Pregnant women infected with the Zika virus during the first trimester of pregnancy have a risk of up to 13% that their infants will be born with microcephaly, according to an online report in the New England Journal of Medicine. Scientists at the Centers for Disease Control and Prevention (CDC) and Harvard University calculated the risk based on Zika infection data, as well as on cases of infants born with microcephaly in French Polynesia, which experienced an outbreak in 2013–2014, and in the hard-hit Brazilian state of Bahia.
The researchers found that the estimated risk of microcephaly due to Zika virus infection during the first trimester of pregnancy ranged from a low of 0.88% to a high of 13.2%. They also reported a negligible association between Zika infection and microcephaly during the second and third trimesters.
Normally, microcephaly is rare in the United States, affecting only between 0.02% and 0.12% of all births. In Brazil, 1,271 babies have been born with the deformity since the Zika outbreak began there last year.
The researchers urged pregnant women to take precautions to avoid Zika infection and for health care systems “to prepare for an increased burden of adverse pregnancy outcomes in the coming years.” The virus, which usually triggers only mild, flu-like symptoms, is mainly spread by two species of Aedes mosquito but can also be transmitted through sexual contact. There is no vaccine to prevent infection.
The authors concluded: “Although much remains unknown about the effects of ZIKV [Zika virus] infection during pregnancy, population-level data from French Polynesia and Bahia reveal a clear association between first-trimester ZIKV infection and microcephaly risk. The pattern was probably similar in other parts of northeastern Brazil, where Zika outbreaks in early 2015 were followed by microcephaly outbreaks in late 2015. If the risk of infection and adverse outcomes is similar in the other geographic areas where ZIKV has since spread, many more cases of microcephaly and other adverse outcomes are likely to occur.”