“I think we were all aware that Zika causes brain abnormalities, but it’s been more generic,” said Dr. Rita Driggers, an associate professor of gynecology and obstetrics at Johns Hopkins University School of Medicine, who was not involved in the study. “Now we know more specifically what we’re looking for in terms of brain abnormalities before the microcephaly occurs.”

Together, the images provide a more detailed guide that might help doctors diagnose Zika-related fetal damage earlier — possibly in the second trimester at a point early enough to help women decide whether to terminate a pregnancy, said Dr. Adre du Plessis, director of the Fetal Medicine Institute of Children’s National Health System, who was not involved in the study.

At the same time, the study may eventually help doctors rule out damage caused by Zika infection. “If there’s any uncertainty on ultrasound, we’re concerned that couples that are not risk-takers and don’t want to gamble might be terminating perfectly normal babies, which is of course a concern to us,” he said. “So there is a lot riding on being able to image accurately.”

One finding that surprised several experts could become an especially meaningful diagnostic clue. Many infections that target the brain produce clumps of calcium, called calcification. But in Zika-infected babies, calcification often occurred in an unusual place: at the intersection of the gray matter of the outer layer of the brain, the cortex, and the white matter of the layer just below that.