The Zika virus is causally linked to microcephaly, the birth defect that leads to abnormally small head size in infants, the U.S. Centers for Disease Control and Prevention declared Wednesday in The New England Journal of Medicine. The agency said it has not found any definitive new evidence but has weighed the accumulating data connecting the two conditions and concluded that it was solid enough to call causative.

The CDC pointed to compelling evidence from Brazil that highlights a suspicious temporal relationship between pregnant mothers becoming infected with the virus and their babies being born with microcephaly or other serious brain abnormalities. Researchers from the agency also weighed the biological plausibility of the connection, evaluating studies that found the virus in the brain tissue of affected fetuses and infants.

Another deciding factor for them was the presence of microcephaly and other brain abnormalities in fetuses or infants who had presumably encountered the virus in utero—conditions that are otherwise relatively rare in the general population.

Although the agency had already issued numerous travel alerts advising pregnant women to stay away from areas with Zika transmission, until now the link between the mosquito-borne virus and and various birth defects had only been considered strong —not definitive. Stating the connection definitively, the CDC staff wrote, “might lead to improved understanding of and adherence to public health recommendations.”

The CDC is not issuing any new health recommendations. Instead, it reiterated its advice that pregnant women should take steps to avoid getting bit by mosquitoes. Because Zika can also be sexually transmitted, the agency says couples should use condoms for at least six months after a male partner’s Zika symptoms arise. There is no current evidence that women can transmit the virus via sexual contact.

For more on Zika, see Scientific American’s Special Report.