CHICAGO (Reuters) – Rates of microcephaly and certain other birth defects were 20 times higher in pregnancies affected by Zika compared with pregnancies in years before the virus arrived in the Americas, U.S. researchers said on Thursday.

The increase emphasizes the ongoing risk of Zika during pregnancy, researchers from the U.S. Centers for Disease Control and Prevention said.

The study, published March 2 online in the CDC’s Morbidity and Mortality Weekly report, examined rates of birth defects in Massachusetts, North Carolina and Georgia in 2012-2013 – before Zika’s arrival in the Americas. They tracked the number of birth defects commonly seen among Zika-affected babies, including brain abnormalities and small head size or microcephaly, eye defects and other central nervous system problems.

During those years, birth defects on that list occurred in about 3 of every 1,000 births. They compared this with published rates of infants from a 2016 U.S. Zika registry and found the rates of these same birth defects were 20 times higher, occurring in nearly 60 of every 1,000 completed pregnancies with Zika infections.

The CDC continues to recommend that pregnant women avoid travel to areas with Zika, and pregnant women living in these areas take steps to protect themselves from infection.