Mothers who are infected with Zika in the third trimester of their pregnancy won’t have babies with noticeable brain defects, researchers announced today in a study published in the New England Journal of Medicine. Birth defects are linked to the virus, but today’s study shows that the time at which a mother is infected is key. The risk of having a child with a brain deformity is much higher if the mother is infected early in the pregnancy.

None gave birth to babies with visible brain abnormalities

The study is based on a national surveillance program in Colombia, which has recorded over 65,000 cases of Zika in the country since the beginning of the outbreak. By sifting through the data, the researchers identified 616 pregnant women who were reportedly infected with Zika in their third trimester. The scientists found that despite those infections, none gave birth to babies with visible brain abnormalities, including microcephaly. Because the outbreak in Colombia is relatively recent, the researchers did not compare the findings to women who were infected in their first and second trimesters; many of those pregnancies are still ongoing, the researchers write.

Two months ago, the CDC confirmed that the Zika virus can cause microcephaly, a birth defect that leads to babies being born with abnormally small heads. But even now, scientists aren’t sure how Zika causes these abnormalities or when an infection is likely to lead to microcephaly. That has a lot to do with the fact that scientists are playing catch-up with the virus, which has spread throughout South and Central America in the past year. But finding and tracking mothers who have been infected with Zika — a virus that resembles the flu and only causes symptoms in 20 percent of those who are infected — can also be hard. Now, scientists are starting to publish the studies they’ve been working on, and as they trickle in, we’re getting a better sense of the risk associated with Zika during pregnancy.

Today’s finding marks an important contrast with findings regarding the Zika infections in the first trimester. One study, for instance, found that the risk for microcephaly can be as high as 13 percent if a mother is infected in her first trimester. That study also found that there was "a negligible association" between birth defects and Zika infection in the second and third trimesters. "Their conclusions make sense from what we know so far, that early pregnancy, the time in pregnancy when the brain and other organs are forming, is most vulnerable," says Lee Norman, an intelligence officer in disaster medicine planning in the United States Army National Guard who didn’t work on the study. "We would all feel more comfortable if the sample size were larger, of course, but this should become more elucidated over time."

"Their conclusions make sense from what we know so far"

Ernesto Marques, an infectious disease expert at the University of Pittsburgh who didn’t participate in the study, agrees. "For me and to most clinicians, the fact that there is a greater risk of a visible anatomical abnormality in infections during the first trimester is not going to be a surprise." That said, Marques would like to see results from more studies. He also points out that even though the babies in today’s study were born without anatomical abnormalities, they could still have cognitive problems. But that remains to be seen, Marques says.

In most cases, the Zika virus isn't dangerous. But for people who are thinking of having kids, finding out that the virus has been linked to stillbirths, problems with the placenta that may harm a developing fetus, and microcephaly can be concerning. Right now, there's no cure or treatment for the virus. And even though most Zika infections happen because of a mosquito bite, it is possible to become infected through sexual contact. That’s why the US government recommends that women with Zika should wait eight weeks before trying to conceive, and men with symptoms should wait at least six months.