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new study of one of the first known Zika outbreaks suggests that women infected with the virus during the first trimester of their pregnancies face a 1 in 100 chance of delivering a child with an abnormally small head, a condition known as microcephaly.

Researchers combed the medical records from French Polynesia, which experienced an outbreak in 2013 and 2014, looking for cases of microcephaly. Using mathematical modeling, they determined that women there had only a 1 percent chance of giving birth to a microcephalic child, according to the study, which was published Tuesday in the journal the Lancet.

The normal rate of microcephaly, though, is estimated at between 2 and 12 cases per 10,000. Extrapolating from the data in the new study would suggest 95 microcephaly cases among 10,000 women infected in the first trimester of pregnancy.

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Although a 1-in-100 incidence seems rare, the researchers — from the Institut Pasteur in Paris and elsewhere — focused just on microcephaly, now seen as only part of the picture when it comes to the damage Zika can wreak in the womb. A recent study by American and Brazilian scientists reported that abnormalities were seen in 29 percent of fetuses carried by a group of women who were confirmed to have been infected with Zika during pregnancy.

“Microcephaly seems to be the extreme,” said Dr. Karin Nielsen-Saines, the senior author of that paper, which was published in the New England Journal of Medicine in early March.

“I personally thought that microcephaly was just the tip of the iceberg, that there was a whole host of conditions associated with this infection — which makes sense with all we know about congenital infections,” she said. “There’s never only one finding. There’s always a syndrome, you know? Many things.”

Simon Cauchemez of the Institut Pasteur’s emerging diseases epidemiology unit, the lead author of the new study, acknowledged the finding of 1 in 100 women seems low, especially seen in the light of Nielsen-Saines’ recent paper. He suggested the variation in results probably relates to differences in the studies. Among them: Nielsen-Saines and her colleagues tracked the pregnancies of Brazilian women who were known to be infected with Zika.

Nielsen-Saines, who was not involved in Cauchemez’s study, suggested the microcephaly rate “looks low for what it is in Brazil.”

“I think if you’re just looking at measurements in … a medical record, that’s very different from doing real-time imaging of the [fetal] brain, and then having the babies born and being able to assess,” said Nielsen-Saines, a professor of pediatric infectious diseases at the David Geffen UCLA School of Medicine.

Her study is continuing to enroll pregnant women in Brazil who were infected with Zika; they hope to eventually follow 600. “And if the numbers aren’t as bad as we published originally … we’ll know.”

Cauchemez told STAT he believes the risk may be greater for women who had symptomatic infections, versus those who were infected but had no symptoms. It is thought that 4 out of every 5 people infected with Zika show no signs of illness.

“It’s still very noisy. But the kind of signal I get from the different studies is that there may be a real difference between risk in symptomatic women and those that are not symptomatic,” he suggested in an interview.

Laura Rodrigues, who has been working in Recife, Brazil on the Zika investigations there, said that might explain part of the difference, but she still thinks Cauchemez’s findings underestimate the problem.

In a commentary Rodrigues wrote for the Lancet, she used Nielsen-Saines’ data to suggest that the risk of microcephaly after a first trimester infection might be as high as 22 percent. Rodrigues teaches at the London School of Hygiene and Tropical Medicine.

She and Cauchemez agreed that the true picture is still coming into focus, and there will be more evidence on which to make such estimates soon. Nielsen-Saines and her group plan to issue an update on the women they are following — the cohort now has 300 pregnant women registered — in the next two months. Other groups in Brazil and Colombia will also be reporting results.

Likewise, Cauchemez noted that, as Nielsen-Saines reported, there is a spectrum of neurological health problems in babies born to women who were infected with Zika during pregnancy. And problems are evident even when the women were infected in the second and third trimesters.

“It’s not saying ‘If your child isn’t microcephalic, he’s fine.’ There could be other complications,” he noted.

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