Women who gave birth to babies with birth defects in Brazil last year were often in their first trimester during the Zika outbreak, according to a report released yesterday by the US Centers for Disease Control and Prevention. The finding provides evidence that getting infected with Zika early in a pregnancy may increase the risk of a baby being born with an abnormally small head, a condition called microcephaly.

In 2015, 574 babies were born with microcephaly in Brazil — more than triple the average number before the outbreak, which was about 157 a year. To find out if the Zika virus might be responsible, researchers examined these pregnancies and counted backward to identify the mothers' first trimester. Then, they looked at how these matched up with the occurence of Zika-like illness in Brazilian states.

Cases of microcephaly more than tripled in 2015

The researchers found that a significant number of people reported Zika-like symptoms during the first trimester of these pregnancies. In addition, they found that the prevalence of microcephaly was a lot higher in states with confirmed cases of Zika transmission compared with states without.

There’s no vaccine or treatment for the Zika virus. But for most people, Zika infections are relatively harmless. Only 20 percent of infected persons develop symptoms, which last around a week and are similar to the flu. However, in 2015, Brazilian authorities linked an outbreak of Zika with a sharp rise in the number of babies born with microcephaly that year. Since then, scientists have been scrambling to figure out if Zika causes birth defects. To definitively prove a link, researchers need more information, Margaret Chan, the WHO's director-general said in a press conference yesterday. Yesterday's report is a valuable data point, drawing a connection between Zika infections during pregnancy and microcephaly.

The study has some limitations. For one thing, microcephaly was probably underreported in Brazil before the Zika virus outbreak, the researchers write in the study. Now that doctors are looking for it, they may also be finding more cases that aren’t Zika-related. Still, there really does seem to have been a sharp rise in the number of children with underdeveloped brains last year — even if it wasn’t as big as this study suggests. In addition, because the association between Zika and microcephaly is a recent one, the researchers didn't have access to much lab evidence confirming a Zika virus infection for the pregnancies they examined. Doctors simply weren't paying attention to the disease at the time, so women weren't getting tests to confirm the illness as often as they are today.

Microcephaly was probably underreported in Brazil

Still, the study's results warrant a warning: women who live in areas with Zika should protect themselves from mosquito bites, the authors write. In addition, the WHO has advised pregnant women who don't live in areas with Zika to avoid traveling to regions where the virus is spreading. So far, only two countries — Brazil and French Polynesia — have reported a significant rise in cases of microcephaly. But the World Health Organization reported yesterday that intense surveillance for fetal abnormalities are underway in other countries.