Pregnant women infected with the Zika virus during their first trimester face as high as a 13 percent chance that their fetus will develop a severe and rare brain defect, according to research published Wednesday.

That condition, known as microcephaly, is characterized at birth by an abnormally small head and often incomplete brain development. Researchers at the Centers for Disease Control and Prevention and Harvard T.H. Chan School of Public Health identified the sharply higher risk after analyzing data from one of the hardest hit areas in Brazil, the epicenter of the rapidly evolving Zika outbreak.

Typically, microcephaly occurs in .02 percent to .12 percent of all births. Even more common congenital conditions, such as Down syndrome, are often seen in less than 1 percent of births. By contrast, the study published in the New England Journal of Medicine found the estimated risk for microcephaly with Zika infections in the first trimester of pregnancy ranged from 1 percent to 13 percent.

The analysis is the first to quantify such risk in pregnant women infected during the current outbreak, which has seen the mosquito-borne virus spread to more than 40 countries and territories in the Americas and beyond. The latest tests showed the same strain is now on the African archipelago of Cape Verde.

“It is an appreciable risk,” said Michael Johansson, a CDC biologist and lead author of the study. “We need to do whatever we can to help women avoid Zika virus infections during pregnancy.”

The study comes just weeks before the start of summer and mosquito season across the United States. The CDC and local public health officials, particularly in the South and Southwest, are highly concerned about many communities’ ability to track and prevent spread of the virus.

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