Researchers say a severe birth defect caused by a 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.

The Centers for Disease Control and Prevention released the findings Tuesday.

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.

The 13 babies in this study had normal head sizes at birth, but 11 of them were later diagnosed with microcephaly.