(CNN) The Zika virus causes microcephaly and other birth defects, the U.S. Centers for Disease Control and Prevention said Wednesday.

"This study marks a turning point in the Zika outbreak. It is now clear that the virus causes microcephaly," CDC director, Dr. Tom Frieden said.

Previously, the agency said it's likely the virus in pregnant women was the cause of the rare birth defect that results in an underdeveloped brain and that the evidence was mounting. However, they maintained that more research was needed before they could conclusively say it is causal.

There was no smoking gun that lead to this proclamation, according to a special report detailing the evidence published online by the New England Journal of Medicine Wednesday.

Based on all of the available evidence, the CDC said two separate sets of criteria to determine a pathogen or environmental exposure causes a birth defect have been met.

"We started using criteria about a month ago to see which ones had been met and which ones had not been met. We wanted to do this in a systematic and calculated way." said Dr. Sonja Rasmussen, lead author of the report and editor-in-chief of CDC's Morbidity and Mortality Weekly Report.

There was also no alternative explanation to account for the increase in these congenital defects among women who had the Zika virus during pregnancy. Frieden said it's like putting a puzzle together.

Last week the World Health Organization said the mosquito-borne virus causes microcephaly and Guillain-Barré syndrome. The CDC however, said they are not yet ready to conclude the virus causes Guillain-Barré syndrome. More than 1,000 cases of microcephaly and other fetal malformations believed to be Zika-associated have been reported from six countries, according to the WHO

Rasmussen cautioned that there are still many unanswered questions.

For example, not all babies born to mothers who had Zika virus while pregnant will have babies with birth defects, but the amount of risk is still unknown. Another outstanding question: Does the stage of pregnancy at the time of infection impact the risk or outcome?

"We have studies that are ongoing that we hope will answer these questions as soon as we possibly can," she said.

Also unknown is the full range of health problems that can result from the virus. Cases of Zika-related microcephaly and congenital birth defects appear to be more severe than what pediatricians see from non-Zika related microcephaly based on clinical reports.

These include smaller head measurements than expected and a condition called fetal brain disruption sequence in which the virus has a destructive effect on the brain.

Reinforcing the CDC announcement, researchers released a new study looking at brain scans of babies born with suspected Zika related microcephaly in Pernambuco, Brazil, between July and December of last year. They looked at MRI and CT scans of 23 babies and found most of them had severe brain damage they characterized as "extremely severe" and an indication of "poor prognosis of neurological function."

The study is published in BMJ, the British Medical Journal.

Earlier outbreaks of the Zika virus, in the Pacific Islands, did not result in adverse reports related to pregnancy. More than 50 years ago, the rubella virus was identified as the cause of an epidemic of congenital defects but no other infectious pathogen has been linked this way since then, which is why the CDC said their approach was "cautious."

"This is an unprecedented situation," Frieden said. "Never before in history has there been a situation when a bite from a mosquito can result in such a devastating scenario."

This does not change any of the recommendations or guidance previously issued by the CDC.

"We know mosquito bites spread other diseases as well, so it's important for pregnant women, and everyone, to not get bitten by mosquitoes," Rasmussen said. That means wearing long pants and long sleeves when outside, using mosquito repellant and removing any standing water from around homes and throughout communities.

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CDC teams have been proceeding as if this was the case even without the official conclusion, but now Rasmussen hopes this will not only raise awareness about prevention but also add focus to ongoing Zika research.

Earlier in the day, the White House said President Barack Obama will sign a bill that offers incentives to companies working on Zika treatments and vaccines. White House press secretary Josh Earnest said the bill is positive but meager, in reference to the pending request to Congress for $1.9 billion in emergency Zika funds.

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