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new study may add weight to the coalescing belief that the Zika virus is most dangerous to a developing fetus in the first trimester and early in the second.

The analysis, by scientists from the US Centers for Disease Control and Prevention, used data from a Zika outbreak in the northeastern Brazilian state of Bahia to determine when during pregnancy Zika-infected mothers who gave birth to children with abnormally small heads likely contracted the virus.

The scientists also used the Bahia data to design a tool that will help other Zika-affected jurisdictions project when they might start to see babies born with microcephaly or other brain-related birth defects.

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A surge in births with these types of developmental problems will place a huge burden on public health services, so having a better sense of the timing is critical, said Dr. Christina Chambers, codirector of the Center for Promotion of Maternal Health and Infant Development at the University of California, San Diego.

“To try to be able to make projections about when you would see these cases show up, I think, is really important,” said Chambers, who was not involved in the work. But she cautioned against assuming that the first and early second trimester are the only periods in which maternal infection can cause fetal defects.

The fact that, to date, only Brazil has reported an increase in cases of microcephaly has led some to question whether Zika infection during pregnancy is truly linked to this birth defect.

Health authorities in French Polynesia also reported a rise in cases of microcephaly after their Zika outbreak in 2013-2014, but the increase was only noted through a review of medical records undertaken when Brazil raised the alarm last fall.

Some experts have suggested the lack of evidence from elsewhere is simply a matter of timing. Other countries in Latin America and the Caribbean only started to see Zika cases months after the virus arrived in Brazil, so women who were infected during pregnancy in those areas haven’t yet given birth.

CDC projections for when to expect babies born with microcephaly after a Zika outbreak.
CDC CDC projections for when to expect babies born with microcephaly after a Zika outbreak.

The new paper from the CDC used the Bahia outbreak to draw up projections. If an outbreak began in a country last November, the authors said, babies who might have been infected in the womb would be born between April and September.

Knowing when a Zika infection during pregnancy would be most dangerous to the fetus will permit public health officials to tailor their advice to pregnant women, said senior author Margaret Honein, an epidemiologist and cohead of the pregnancy and birth defects task force in the CDC’s Zika response.

Honein also stressed the need to learn more about the full range of birth defects being seen among Brazilian babies whose mothers were infected during pregnancy. In recent weeks, it has become clear that microcephaly is just one birth defect among many that are being reported in newborns there.

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