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 got 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.