Women have up to a 13 percent chance of their babies being born with the birth defect microcephaly from the Zika virus, according to a new study.

The study published in the New England Journal of Medicine Wednesday estimated the risk of getting microcephaly from Zika ranged from just under 1 percent to 13 percent. The study also found that the most vulnerable time to get Zika is in the first trimester of pregnancy, as the risk of the birth defect falls off in the second and third trimesters.