By Julie Steenhuysen

CHICAGO (Reuters) - Thirteen babies in Brazil born with normal head circumference have been diagnosed with congenital Zika syndrome, with brain scans showing extensive malformations, inflammation and reduced brain volume, researchers reported on Tuesday.

Of the 13 infants, 11 gradually developed the birth defect microcephaly, or abnormally small head size, in the months following birth.

The findings raise new concerns about the hidden effects of pre-natal exposure to the mosquito-borne Zika virus, which has been shown to cause birth defects when women become infected during pregnancy.

On Friday, the World Health Organization declared the global Zika emergency over because the link between Zika and microcephaly has been confirmed. WHO intends to continue studying Zika as a serious infectious disease that will require years of research.

Although others have observed neurological problems in infants exposed to Zika during gestation, the study is the first to carefully document birth defects in a group of babies with confirmed Zika exposure whose head circumference fell into the normal range at birth.

The study, published on Tuesday in the U.S. Centers for Disease Control and Prevention's weekly report on death and disease, was done by teams in Recife and Fortaleza in northeastern Brazil.

Eleven of the infants were born with heads that were on the small side and were referred for evaluation shortly after birth. The remaining two, born with normal head circumferences, were referred for evaluation at 5 to 7 months because of developmental concerns.

Among the observed symptoms, 10 of the 13 babies had trouble swallowing, seven had epilepsy, five showed some degree of irritability, nine could not voluntarily move their hands and all had hypertonia, or excessively stiff muscle tone.

In a conference call with reporters on Tuesday, WHO officials said the fact that children can be born with normal head size but later develop microcephaly demonstrates that the definition of congenital Zika virus syndrome - the term WHO has associated with Zika-related birth defects - continues to expand.

Dr. Anthony Costello, WHO's expert on maternal, newborn, child and adolescent health, said some 2,100 babies in Brazil have had confirmed cases of microcephaly related to Zika. He expects another 1,000 cases to be confirmed as doctors continue to investigate a backlog of suspected cases.

"We know the problem has not gone away in Brazil," he said.

(Reporting by Julie Steenhuysen; Editing by Dan Grebler)