Scientists zero in on microcephaly risk for babies born to Zika-infected mothers

If a pregnant woman contracts Zika, how likely is her fetus to develop an abnormally small head?

Scientists don’t yet know, but they might be getting closer to an answer.

On Wednesday, researchers put the risk at between 1 percent and 13 percent when the infection occurs in the first trimester of pregnancy. A previous study, based on data from a Zika epidemic in French Polynesia in 2013 and 2014, concluded that 1 percent of infected pregnant women carried a fetus that developed an abnormally small head, a condition known as microcephaly.

Although those figures may seem low, researchers in both cases were examining only the risk of microcephaly. Zika virus has been tied to a host of birth defects; researchers following the pregnancies of infected women in Rio de Janeiro found that 29 percent had babies with brain-related birth defects, including but not limited to microcephaly.

The latest study was published online Wednesday by the New England Journal of Medicine.

The authors, from the Centers for Disease Control and Prevention, examined data from the Zika outbreak in Bahia, a state in eastern Brazil. Because there are still so many unknowns when it comes to Zika, their calculations involved a series of different kinds of permutations.

For instance, it is not known what percentage of people in Bahia were infected with the virus. In French Polynesia, studies after the fact suggested more than 70 percent of people were infected. A 2007 outbreak in Yap is estimated to have infected 66 percent of that island’s population.

But outbreaks on islands with small land masses could well behave differently than outbreaks on a continent where communities may be more spread out.

So Michael Johansson, who works at the CDC’s dengue branch in San Juan, Puerto Rico, and his coauthors calculated rates of microcephaly for multiple scenarios.

If 10 percent of Bahia’s population was infected, the microcephaly rate was highest, around 13 percent after infection in the first trimester. But if 80 percent of the population had contracted Zika, the risk was close to 1 percent.

The authors also tried to make allowances for the fact that not all reported cases of microcephaly turn out to be verified, and not all cases of the potentially devastating birth defect are caused by Zika infection.

Microcephaly can be caused by a number of factors, including a variety of viral infections and alcohol consumption during pregnancy. In the US, between two and 12 babies per 10,000 are born with the condition.

The CDC group’s work suggested the risk was highest in the first trimester of pregnancy and negligible in the second and third. But Johansson said that doesn’t mean infection later in pregnancy doesn’t carry risks.

“I want to be clear that there is risk [of microcephaly], at least in the second trimester,” he told STAT in an interview.

Dr. Christina Chambers, who was not involved in the work, called it a thoughtful analysis, but said more facts are needed to get to firm answers about Zika’s risks during pregnancy.

“These estimates are based on a number of assumptions that can’t yet be verified,” Chambers, codirector of the Center for Promotion of Maternal Health and Infant Development at the University of California, San Diego, said in an email.

She added: “Clearly infection in any trimester seems to be associated with microcephaly so as they conclude, should be avoided. It might make sense that first trimester carries the greatest risk but [that] remains to be seen as we get more individual level data.”