Aug. 8, 2016 -- CDC researchers have made a startling discovery about microcephaly, a devastating birth defect caused by the Zika virus: It can affect babies late into a mother’s pregnancy and not become apparent until months after they are born.

Up to now, researchers have said babies were most susceptible to the virus when a woman was infected early in a pregnancy. Babies who have the devastating condition generally have abnormally small heads at birth.

But in the newly described form of microcephaly -- dubbed late-onset microcephaly -- babies appear to have normally sized heads at birth, but their damaged brains have stopped growing. By about 6 months of age, they have developed microcephaly because their head size hasn't kept up with normal growth, said William Dobyns, MD, professor of pediatrics and neurology at Seattle Children's Hospital. Dobyns has been studying brain injuries in Zika-infected babies.

The condition was observed by CDC researchers who are following more than 1,200 Zika-affected pregnancies in Brazil, said Ted Pestorius, an assistant incident manager for the CDC's Zika response. The finding has not been published in a medical journal.

Previously, scientists have described a fetal brain defect associated with Zika that causes the brain to shrink and the skull to collapse while a baby is still in the womb. Published case reports have also described a kind of late-onset microcephaly that doctors can’t detect on ultrasound scans but diagnose shortly after birth.

Pestorius shared the discovery with media attending a Zika briefing arranged by the Georgia Department of Economic Development on Wednesday. The CDC did not immediately provide more details about the study.

Cesar Victora, MD, PhD, an epidemiologist with the Universidade Federal de Pelotas, in Rio Grande do Sul in Brazil, and part of the team that’s been carefully documenting the outcomes of babies born to Zika-infected mothers in Brazil, confirmed that these cases were occurring. He said his colleagues have treated some of the babies.

The babies with late-onset microcephaly have been born to mothers infected during the third trimesters of their pregnancies, Pestorius said.