WASHINGTON • Pregnant women infected with the Zika virus during the first trimester have a risk of up to 13 per cent that their infant will be born with the microcephaly birth defect, a new study says.
Scientists calculated the risk based on Zika infection data and cases of infants born with abnormally small heads and brains in French Polynesia, which experienced an outbreak in 2013 and 2014, and the hard-hit Brazilian state of Bahia.
Brazil has experienced an explosion of Zika infections and cases of microcephaly since 2015.
The study, which appeared on Wednesday in the New England Journal of Medicine, said the risk of microcephaly was very low for pregnant women who contracted the disease beyond the first trimester.
Researchers from the US Centers for Disease Control and Prevention and Harvard University wrote that they found "a strong association between the risk of microcephaly and infection risk in the first trimester" of pregnancy, but "a negligible association in the second and third trimesters".
The researchers urged pregnant women to take precautions to avoid Zika infections, "and for healthcare systems to prepare for an increased burden of adverse pregnancy outcomes in the coming years".