Dr. Vanessa van der Linden, another author of the study and a neuropediatrician at the Association for Assistance of Disabled Children in Recife, Brazil, where most of the babies in the study are patients, said the type of brain damage in the babies who later developed microcephaly “presented the same pattern, but less severe” than those with it at birth.

The babies in the study published this fall also appeared to have a pattern of similar, but less severe, brain damage, said Dr. Antonio Augusto Moura da Silva, of the Federal University of Maranhão and an author of that study, which was published in Emerging Infectious Diseases. He and his colleagues studied 48 babies with brain abnormalities in the northeastern state of Maranhão, identified six babies who did not have microcephaly at birth, and found that three of them later developed it.

“We were worried, but now that we’ve started following those cases, we are very sad,” Dr. Silva said. “The picture is really terrible. At the least, if they have microcephaly, we expect them to have a very poor quality of life.”

Experts and the authors of the studies said it was unclear why these infants’ brains did not develop enough to match their age and body size. Dr. Ernesto T. A. Marques Jr., an infectious disease specialist at the University of Pittsburgh and the Oswaldo Cruz Foundation in Recife, who was not involved in either study, said it could be that because of the initial fetal brain damage, “the necessary pathways and hormones that organize growth of the neonatal brain are not there anymore and the brain doesn’t grow.”

It could also be the result of the immune system responding to the original Zika virus infection. Dr. Moore said that another possibility might be that there was still some infection that continued to damage the brain. But she said that seemed less likely, given that follow-up tests for Zika virus conducted on seven of the babies did not find evidence of active infection.