Babies born looking normal and healthy after a Zika infection are developing smaller-than-normal heads months later, doctors reported Tuesday – a bad sign that shows the virus continues to damage a baby for weeks or months.

That means that if a pregnant women gets infected with Zika, her baby could be at risk not only in the womb, but long after birth.

Assessment of a Series of 13 Infants with Congenital Zika Virus Infection without Microcephaly at Birth CDC

And it’s one more unpleasant surprise from the Zika virus, once thought unremarkable and now showing it can wreak havoc in ways never seen before.

Dr. Vanessa Van der Linden, the pediatric neurologist in Recife, Brazil, who sounded the first public alert about Zika, examined 13 babies that appeared to have been infected with Zika in the womb but who were born looking unaffected.

“This report documents that microcephaly at birth is not an essential hallmark of congenital Zika syndrome."

It turns out they were not unscathed.

“Among all infants, head growth was documented to have decelerated as early as 5 months of age, and 11 infants had microcephaly,” Van der Linden and colleagues report in a report from the U.S. Centers for Disease Control and Prevention.

Assessment of a Series of 13 Infants with Congenital Zika Virus Infection without Microcephaly at Birth

Zika virus has been proven beyond all doubt to cause microcephaly, the medical name for a smaller-than-normal head. It’s caused when the brain of a developing fetus is damaged and shrinks, causing the skull to either collapse or to grow slowly, also.

Related: Babies Show a Range of Problems After Zika

It also causes other damage to the brains and bodies of babies.

Van der Linden’s team has been watching many babies born after the mothers tested positive. They focused on 13 in particular.

“All infants had positive tests for Zika virus,” they wrote.

The children had other problems, also. Ten had trouble feeding, the researchers reported. Van der Linden has reported this before, and says Zika-affected babies often suffer from painful reflux. Seven had epilepsy – another common diagnosis – and almost all of them had a stiffening of the limbs called hypertonia.

They make good eye contact, which can lead parents to believe they aren’t as badly affected as they really are, Van der Linden says.

Related: Zika Birth Defects the Tip of the Iceberg

“This report documents that microcephaly at birth is not an essential hallmark of congenital Zika syndrome,” the team wrote.

Assessment of a Series of 13 Infants with Congenital Zika Virus Infection without Microcephaly at Birth. CDC

“Infants with normal head circumference at birth have brain and other abnormalities associated with congenital Zika syndrome and might develop microcephaly after birth. These findings demonstrate the importance of early neuroimaging for infants exposed to Zika virus prenatally and the need for comprehensive medical and developmental follow-up.”

It’s not clear what’s happening. Zika could be causing a long-term infection in the brain or it could damage immature cells in the womb, that then do not go on to develop into normal tissue as they should as the baby grows after being born.

"These findings demonstrate the importance of early neuroimaging for infants exposed to Zika virus prenatally and the need for comprehensive medical and developmental follow-up.”

Zika has spread across Latin America, the Caribbean and is causing smaller outbreaks in south Florida. Some countries have reporting matching epidemics of microcephaly and other birth defects.

Related: Zika Affected Woman's Memory

In the United States, the CDC reports more than 4,200 cases of Zika, nearly all carried by travelers from other areas. Florida reports more than 200 locally-acquired cases, spread by the Aedes aegypti mosquitos that thrive in Florida.

Florida officials cleared on area of Miami Beach where Zika had been spreading.

“The newly cleared area, which is about three miles, is between 28th to 63rd streets. The last known person contracted Zika in this area on September 27th,” the state health department said in a statement.

“The remaining area of active Zika transmission in Miami Beach is about 1.5 square miles between 8th and 28th streets.”

Related: Images Show Zika's Destruction of Babies' Brains

In Florida, 160 pregnant women have been infected with Zika. More than 1,000 pregnant women in the U.S. have been infected. Health experts say their babies will need to be monitored for years.

There’s no cure for Zika and the brain damage it causes is permanent.

The CDC says pregnant women or those who could become pregnant should avoid Zika zones.

"All of Miami-Dade remains a yellow area and pregnant women are eligible for Zika virus testing. All pregnant women in the United States should be evaluated for possible Zika virus exposure during each prenatal care visit," the CDC advises.