Overall, there were 9.6 cases of microcephaly per 10,000 live births in Colombia, where the virus infected as many as 20,000 pregnant women
Cases of microcephaly in Colombia were four times higher this year than last, an increase that coincides with a widespread outbreak of Zika virus in the country, a report released on Friday said.
At its peak in July, microcephaly cases in Colombia were nine times higher than in the same month in 2015, according to the U.S. Centers for Disease Control and Prevention's weekly report on death and disease.
Overall, there were 9.6 cases of microcephaly per 10,000 live births in Colombia, where the virus infected as many as 20,000 pregnant women since the start of the outbreak there in October 2015, according to the report by researchers at the CDC and the Colombian health department.
The numbers reflect a sharp increase in rates of the rare birth defect, proving that microcephaly was not occurring primarily in Brazil.
"This finding confirms that countries with Zika virus outbreaks are likely to experience large increases in microcephaly and other Zika-related birth defects," the CDC said in a statement. "The report also suggests that Zika virus infection in the first months of pregnancy poses the greatest risk of microcephaly."
According to the report, which covers the period from August 2015 through November 2016, Colombia had 476 cases of microcephaly, including 432 live-born infants and 44 that occurred among fetuses that did not survive the pregnancy.
The number was far lower than those in Brazil, where Zika arrived in May 2015. As of Dec. 3, Brazil has confirmed 2,228 cases of microcephaly linked with Zika, and is still investigating another 3,173 cases.
The study's authors said the difference could have resulted from a number of factors, including the fact that women in Colombia had early warning about the risk of microcephaly.
In February, the Colombian Ministry of Health advised women to consider delaying pregnancy for six months, which may have played a role. During the study period, the number of live births fell by about 18,000 from 2015 to 2016. Several experts also have suggested that women in Colombia took advantage of more permissive abortion laws, an option that was not available to women in Brazil, where abortion is banned in most instances.
There were other possible differences as well, including the fact that Colombia had a higher baseline number of microcephaly cases than Brazil and that 50-75 percent of Colombians live at altitudes above 2,000 meters, where Zika-carrying mosquitoes are not active.