U.S. health officials say there's no longer any doubt that the Zika virus causes severe birth defects.
Experts had been cautious about making a definitive link despite a surge of babies born with a rare birth defect in Brazil during a Zika outbreak. The virus is mainly spread by mosquitoes, and no mosquito-borne virus had ever been known to cause birth defects.
But on Wednesday, the Centers for Disease Control and Prevention said there's enough evidence now to declare Zika the cause of a birth defect called microcephaly and other brain abnormalities.
While the finding means women infected with the virus during pregnancy have "an increased risk of having a baby with these health problems," it does not mean "that all women who have Zika virus infection during pregnancy will have babies with problems," the CDC said. "Some infected women have delivered babies that appear to be healthy," it added.
CDC officials said their advice to pregnant women won't change. Pregnant women should avoid traveling to places where the Zika virus is spreading, mostly in Latin America and the Caribbean.
The agency is "launching further studies to determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems," said CDC Director Tom Frieden.
The CDC's report, published Wednesday in the New England Journal of Medicine, said "no single piece of evidence provides conclusive proof" that Zika causes microcephaly and other brain defects, the agency said. "Rather, increasing evidence from a number of recently published studies and a careful evaluation using established scientific criteria supports the authors' conclusions."
Confirming the connection between Zika and brain defects "is an important step in driving additional prevention efforts, focusing research activities, and reinforcing the need for direct communication about the risks of Zika," the CDC said.
The agency has joined the Obama administration's effort to persuade Congress to approve $1.9 billion dollars for the fight against the virus.
Speaking at a White House briefing on Monday, CDC Principal Deputy Director Dr. Anne Schuchat said, "everything we look at with this virus seems to be a bit scarier than we initially thought."
This story has been updated.