The US Centers for Disease Control and Prevention (CDC) has confirmed that the Zika virus causes severe birth defects, including microcephaly.
Hundreds of babies were born in Brazil last year with microcephaly, a syndrome where children are born with unusually small heads.
The defects coincided with a spike in Zika infections, leading experts to suspect the mosquito-borne virus.
Research has now affirmed those experts' suspicions, the CDC said.
"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly," said Dr Tom Frieden, the head of the CDC.