Therapist Rozely Fontoura holds Juan Pedro, who has microcephaly, in Recife, Brazil, in March 2016.Paulo Whitaker / Reuters

Zika is beyond a doubt scariest for pregnant women, or for women trying to get pregnant. For women living in areas where the mosquito-borne virus is spreading, who are trying to make the huge decision of whether, and when, to have a child, there’s not a lot to go on. The Centers for Disease Control and Prevention recommends that women who’ve gotten Zika should wait at least eight weeks to conceive, and that men who’ve gotten it should wait at least six months.

But the vast majority of people who are infected with Zika don’t show any symptoms, and children can still be born with birth defects and mothers can still miscarry, even if they never had symptoms. So it’s possible that microcephaly—a birth defect caused by Zika that leads to smaller-than-average head size and other developmental problems—could sneak up on parents who never knew one of them had Zika in the first place.

Margaret Chan, the director of the World Health Organization, summed up the harsh reality of the situation in her recent address to the 69th World Health Assembly: “To protect women of childbearing age, all we can offer is advice. Avoid mosquito bites. Delay pregnancy. Do not travel to areas with ongoing transmission.”

This is not the most reassuring advice. But a new paper published in the New England Journal of Medicine estimates how likely it is that the fetus of a woman infected with Zika would develop microcephaly, offering would-be parents at least some idea of the risk they’d be taking on.

Analyzing current outbreak data from the Brazilian state of Bahia, researchers from the CDC calculated that if a woman is infected in her first trimester, there is between a 1  and 13 percent chance of her fetus developing microcephaly.

That’s a big range! We have Zika’s stealthy nature to thank for that. The first number is based on the assumption that the Zika infection rate in Bahia’s population is 80 percent and that half the cases of microcephaly reported are false. The second presumes a 10 percent infection rate and that every case reported is true. There’s probably been at least some overreporting of microcephaly in Brazil, and since many people don’t show symptoms and don’t get tested, the actual infection rate is hard to pin down. So we don’t know where in that range the truth lies.

A previous analysis of the 2013-2014 Zika outbreak in French Polynesia found about a 1 percent risk—but the microcephaly cases there were found retrospectively, so that might be on the low side.

Some important caveats—this study only looked at first-trimester infection, and infection at all stages of pregnancy has been associated with fetal abnormalities and other pregnancy problems. And it also only looked at microcephaly specifically. “Microcephaly is only one possible adverse outcome among a spectrum of conditions that may be part of congenital Zika syndrome,” the researchers write.

Still, at least we now have a risk estimate to offer women, along with advice.

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