Women infected with the Zika virus late in their pregnancies had babies with no apparent birth defects, according to a study in Colombia that seems to confirm that the greatest risk to infants comes early in pregnancy.

But the study also found troubling cases of severe birth defects in babies born to women who never realized they had contracted Zika.

"You're not out of the woods if you don't have symptoms," said Dr. W. Ian Lipkin, a Columbia University infectious diseases expert who was not involved in the research.

Ever since a Zika virus outbreak in Brazil was linked to severe birth defects late last year, health experts have been trying to understand when developing fetuses are most vulnerable and whether fetuses are at risk if the mother is infected but never experiences symptoms.

Of 1,850 Zika-infected pregnant women the authors tracked, about a third caught the virus late in pregnancy, during the third trimester. Most of those women have given birth, and no cases of microcephaly or brain abnormalities were seen in any of their babies, the researchers found.

The Zika virus is spread mainly through the bite of a tropical mosquito. Most people infected never develop symptoms. Others get a fever, rash, joint pain, or red eyes, and recover within a week.

Brazil, which has suffered the largest outbreak of Zika, has had more than 500 cases of Zika-linked microcephaly, a severe birth defect in which a baby's skull is much smaller than expected because the brain hasn't developed properly.

Disease experts have been watching to see if the epidemic would play out in Colombia, Brazil's northern neighbor, in similar fashion.

The study's authors call the report "preliminary" and most of the women followed by researchers were still pregnant at the time the report was completed. Researchers want to track both the pregnancies and the children who have already been born, said Margaret Honein, of the U.S. Centers for Disease Control and Prevention, who was one of the authors of the study. It's possible, she said, that the babies may still develop problems.

And Colombian officials have only been reporting on live births. They have not said how many Zika-affected pregnancies developed birth defects but ended in stillbirth, miscarriage or abortion.

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It's not surprising that infections later in pregnancy did not lead to apparent defects, because experts know that Zika and other viral infections early in pregnancy are more likely to cause defects like microcephaly, said Dr. Neil Silverman, a UCLA professor of obstetrics who has been advising the California Department of Public Health on Zika issues.

He said the total number of defects out of Colombia appear to be surprisingly low, however. Colombia saw only four confirmed Zika-related microcephaly cases between August and April, when the study was conducted. Two more have been reported since then.

The researchers don't know at what trimester the mothers of the four microcephaly cases were, because none of the women experienced symptoms, Honein said.

That means that while it is unlikely, a third trimester infection could not be completely ruled out in each of those cases.

It also "really heightens our concern" that even women who don't feel sick can pass a Zika infection to their babies, and the babies can develop defects, Honein said. Most Zika-caused birth defects have been reported in mothers who said they developed Zika symptoms while they were pregnant.

The study was conducted by Colombian and American health officials, tracking reports of Zika infections and of birth defects seen between early last August and early April. It was published online Wednesday by the New England Journal of Medicine.