Since the Zika virus first appeared in the U.S. in 2015, there has been only one baby born with Zika-related birth defects in San Diego County, and only five in California.
A new report from the Centers for Disease Control and Prevention says nationwide, 10 percent of babies born to Zika-infected mothers in 2016 had birth defects.
The agency says women who become infected with Zika in their first trimester are at highest risk.
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Dr. Anne Schuchat, the CDC's acting director, said the small number of babies born with Zika-related birth defects may not reflect the full impact of the virus.
“Some seemingly healthy babies, born following pregnancies complicated by Zika, may have developmental problems that become evident months after birth,” she said.
That's why her agency recommends that all babies born to Zika-infected mothers should be closely monitored for developmental problems.
Peggy Honein, Ph.D., co-lead of the CDC's Pregnancy and Birth Defects Task Force, said these babies should get their brains scanned.
“Brain imaging, for example a brain ultrasound or a CT, is important to look for abnormalities, because we know that some babies have underlying brain defects that are otherwise not evident at birth," she said.
The CDC reports last year, nearly 1,000 pregnant women from 44 states had some evidence of recent Zika infection.
The majority of affected women acquired the virus during travel to an area where Zika is present.