NEW YORK — 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 were infected with 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, researchers said.

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, onf of Brazil’s northern neighbors, in similar fashion.

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

The study was conducted by Colombian and US health officials and was published online Wednesday by the New England Journal of Medicine.