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If A Pregnant Woman Gets Zika In Her First Trimester, Microcephaly Odds Are 1 In 100, Study Finds

“I would take this as a low end estimate of the risk,” one scientist said.

Posted on March 15, 2016, at 7:09 p.m. ET

Mario Tama / Getty Images / Via

A Zika infection in the first trimester of pregnancy triggers microcephaly in about 1 in 100 births, suggests a study of the 2013 Zika outbreak in French Polynesia published on Tuesday.

Microcephaly — a birth defect marked by a shrunken brain and skull and, often, a lifetime of disability — has emerged as the most striking signature of a Zika outbreak that began last year in Brazil. More than 4,000 microcephaly cases are under investigation there, and some have been more recently reported in Colombia.

One of the big questions in the outbreak is how often a Zika virus infection in pregnancy triggers these birth defects. The new study, published in The Lancet journal, looks at the 2013 outbreak of the disease in Tahiti and the other islands of French Polynesia, keying on eight cases of microcephaly identified there in the last three years. Public health officials reported nearly 9,000 cases of serious Zika disease there at the height of the outbreak, and perhaps two-thirds of all French Polynesians may have been infected by the virus.

“Our analysis strongly supports the hypothesis that infection in the first trimester of pregnancy is associated with an increased risk of microcephaly,” the study authors, led by Simon Cauchemez of France’s Institut Pasteur in Paris, write. Seven of the eight microcephaly cases followed close after the height of the outbreak. Only infection in the first trimester of pregnancy seemed linked to microcephaly with statistical certainty, the researchers reported, estimated to happen in about 1% of those cases.

That rate is lower than that of other viruses associated with birth defects, such as cytomegalovirus (around 13% of cases lead to birth defects) and rubella (at least 38% of cases). “However, an important difference is that the incidence of Zika virus in the general population can be very high during outbreaks,” the authors write, making Zika’s birth defect risk very worrisome.

“I would take this as a low end estimate of the risk,” Peter Hotez, a professor of molecular virology and microbiology at the Baylor College of Medicine who was not involved in the new study, told BuzzFeed News. A recent New England Journal of Medicine analysis of 88 pregnant women infected with Zika virus, for example, found 29% had fetal abnormalities revealed by ultrasound. “That’s probably the high end of the risk and the new study is the low end.”

Hotez notes that microcephaly is a severe and obvious birth defect, whereas more subtle problems, such as eye damage or learning disabilities, may only become evident in the years after birth.

The study also bolsters evidence that Zika alone triggers brain damage in infants, Hotez added, and not a co-infection with another virus such as dengue, a suggestion made by some public health officials observing the incidence of both diseases in Brazil. “Zika alone really looks like the problem in these birth defects,” Hotez said.

A BuzzFeed News investigation, in partnership with the International Consortium of Investigative Journalists, based on thousands of documents the government didn't want you to see.