Researchers say a severe birth defect caused by Zika infection may not be apparent at birth but develop months afterward, further confirmation that the virus can cause unseen damage to developing babies.

The findings come from a study of 13 Brazilian babies whose heads all appeared normal at birth but then grew much more slowly than normal.

Most people infected with Zika never develop symptoms, but infection during pregnancy can cause devastating birth defects, including microcephaly, in which a baby's skull is much smaller than expected because the brain hasn't developed properly.

Microcephaly is diagnosed based on a measurement of the baby's head circumference. It can be done during pregnancy using ultrasound, or after the baby is born. Doctors then compare the measurement to standard sizes of other kids, based on gender and age.

The study focused on 13 babies born in Brazil late last year and earlier this year. All had heads that were a little small at birth, but within the normal range. Over the next five to 12 months, doctors noted that their heads weren't growing at normal rates. Eleven were eventually diagnosed with microcephaly.

Many of the children also developed other problems that have been linked to Zika, including epilepsy, problems swallowing, muscle weakness and inflexible joints.

Dr. Peter Salama, chief of emergencies at the World Health Organization, told reporters in Geneva last week that understanding of the complications from Zika continues to evolve.

“We are also learning a lot every day,” he said.

The U.S. Centers for Disease Control and Prevention released the findings last week. The authors were a team of researchers from Brazil and the United States.

“This is certainly the first detailed description of these kinds of cases,” said Dr. Ganeshwaran Mochida, a pediatric neurologist at Boston Children's Hospital.

The study confirms that the absence of microcephaly at birth doesn't mean there are no abnormalities in the children of Zika-infected mothers, CDC officials said.

The findings, while discouraging, are not a surprise to scientists. A study out of Brazil earlier this year suggested that one in five cases of microcephaly likely had head sizes in the normal range at birth.

And microcephaly also has been diagnosed months after birth in cases caused by other germs.

Still, it is likely to further worry potentially infected parents who may grow alarmed by signs that their newborn's head is a little small, said Dr. Thierry Huisman, a Johns Hopkins University professor of radiology who has studied Zika-affected children.

The CDC now recommends monitoring babies born to Zika-infected women after birth, but the agency is looking at whether additional imaging should be recommended, said CDC Director Dr. Tom Frieden.

Investigators are working to determine what proportion of Zika-infected women have babies with birth defects, and how the risk varies based on when during the pregnancy the infection occurred. Earlier research has suggested that 1 percent to 14 percent of Brazilian mothers infected in the first three months of pregnancy had babies with microcephaly and that the risk falls when infections happens later in the pregnancy.

The WHO's Salama called the risk “small but significant, but it's definitely a moving target as well.”