(CDC/CNN) - In a new CDC report, researchers looked at 1 million births in 15 U.S. states and territories in 2016. Within that group, 3 out of every 1,000 babies had a birth defect.
Each birth defect was associated with a Zika virus infection in the mother.
Half of those infants were born with brain abnormalities or small head size.
The report singles out Puerto Rico, south Florida and an area in south Texas, which are all areas where transmission of Zika is happening locally.
Those areas saw a 21 percent increase in birth defects linked to Zika in the last half of 2016, compared to the first part of the same year.
Researchers said contributing factors are possible and that the number of birth defects isn't only due to the Zika virus.
The CDC warns that the 2017 data could show another spike in Zika-related birth defects, since women exposed in late 2016 would have given birth the following year.
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