By Alexandra Sifferlin
June 16, 2016

A Zika virus infection may be most risky for pregnant women during the first trimester, according to a top health official.

“It looks like the period of greatest risk is first trimester,” says Dr. Denise Jamieson, chief of the U.S. Centers for Disease Control and Prevention’s Women’s Health and Fertility Branch. “The pattern of infection and outcomes is really consistent across different places.”

As mosquito season approaches in the United States, disease experts are trying to get a clearer picture of the Zika virus’ toll. On Thursday, American health experts reported that three babies in the U.S. have been born with Zika-related birth defects and more cases are possible as the summer continues. But health authorities at the CDC say they are making strides in understanding when during pregnancy women are most at risk.

On Tuesday, Jamieson and other health experts from the CDC and in Colombia reported that around 12,000 pregnant women in Colombia are infected with Zika, but that women who were infected with the virus in the third trimester have had healthy babies so far. However, Jamieson says it’s still too early to know whether these seemingly healthy babies won’t have health problems over time.

“We don’t know the full range of outcomes and there are unanswered questions regarding the babies born apparently healthy,” she says. “Unfortunately we are experiencing the same poor outcomes that we saw in Brazil and we see in Colombia and other places,” Jamieson adds. “We are learning more every day.”

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