Story highlights Brain abnormalities or microcephaly occurred about 33 times as often in pregnancies with Zika infections

New report is based on data from surveillance programs in three US states

(CNN) The proportion of Zika-related birth defects during 2016 was nearly 20 times higher than the number seen during the pre-Zika years, according to a new report from the United States Centers for Disease Control and Prevention.

In particular, brain abnormalities or microcephaly, where a baby's head size is smaller than expected, occurred about 33 times as often in pregnancies with Zika infections than in the pre-Zika years.

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Zika, a virus transmitted primarily by mosquitoes and unprotected sex, has been spreading around the globe with locally transmitted cases in the US were first reported in Florida and Texas during 2016. Currently, there is no vaccine to prevent infection.

Last year the World Health Organization declared a Public Health Emergency of International Concern because of the rapid pace with which the virus was spreading and the alarming number of cases of microcephaly seen in Brazil where large numbers of women had been infected. At the time the link between Zika and microcephaly was still uncertain but has since been proven

"I think women should take that this further demonstrates how important it is to prevent Zika virus infection during pregnancy," said Peggy Honein, epidemiologist and chief of the birth defects branch at the CDC. "These devastating effects have a major increase over the baseline when Zika virus infection occurs during pregnancy."

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