The US Centre for Disease Control and Prevention (CDC) on Wednesday confirmed what has long been suspected about the Zika virus: it causes microcephaly.
The CDC said it reached the conclusion after careful review of the existing evidence, saying it also causes other severe fetal brain defects.
"This study marks a turning point in the Zika outbreak," the CDC said in a news release. "It is now clear that the virus causes microcephaly."
It said the findings support its early guidance to pregnant women and their partners to take steps to avoid Zika infection, and repeated its warning that pregnant women should avoid travel to areas where Zika is actively spreading.
Scientists in Brazil and other Latin American countries, where there are active outbreaks of the Zika virus, have long believed that the virus causes microcephaly, a condition in which babies have abnormally small heads and brains leading to sometimes severe developmental problems.
The CDC also said many questions remained about the virus, which is spread by mosquitos and sexual contact. For example, scientists do not know why some infected women have delivered babies that appear to be healthy.
The CDC is planning to launch 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 Tom Frieden, director of the CDC.
Last month the World Health Organization (WHO) said Latin America could be faced with thousands of neurological birth defects in the wake of current Zika outbreaks. Governments have been urged to expand their health services to care for children in the long term.
In addition to microcephaly in newborns, the virus has been linked to rising numbers of cases of Guillain-Barre syndrome in adults. This neurological disorder can lead to paralysis.
In February the World Health Organization (WHO) declared Zika virus a public health emergency of international concern.