For the first time, US disease experts have agreed that the mosquito-borne Zika virus does indeed cause babies to be born with abnormally small heads — a condition known as microcephaly. Until now, the two conditions were linked — but experts didn’t know for sure if Zika caused the brain-damaged condition. After reviewing existing data about Zika, researchers from the US Centers for Disease Control and Prevention have declared in the New England Journal of Medicine that the relationship was, as suspected, causal.

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

The researchers came to their conclusion by checking existing data against a set of seven rules, known as Shepard's Criteria. The list was developed by a pediatric doctor, Thomas Shepard, as a way to establish if something causes birth defects. When at least some of these criteria are met, it usually serves as "proof" that exposure to a particular agent — such as a disease or some kind of chemical — causes fetal abnormalities.

The data review showed that Zika met the necessary standards for being considered a defect-causing agent. First, many women who get Zika during a critical time of pregnancy — such as the first or second trimester — have babies with birth defects. Additionally, many of the fetuses that are exposed to Zika have followed the same, specific pattern: they are born with abnormally small heads or have other brain anomalies. And finally, many women have given birth to babies with microcephaly after having traveled to areas with ongoing Zika outbreaks. All of this evidence is enough to conclude that Zika causes microcephaly, the researchers argue, though the virus satisfies even more criteria on the list.

The Zika virus has long been thought to cause microcephaly. A recent outbreak of the disease in Brazil and other nearby countries has coincided with a spike in microcephaly cases. More than 1.5 million cases of Zika have been reported in Brazil since the virus's introduction in April 2015, according to the World Health Organization. Since then, reports of microcephaly have spiked as well, reaching somewhere between 4,000 and 5,000, according to Brazil's Ministry of Health. Despite this association, experts have yet to definitively confirm the link between Zika and birth defects.

Now that a causal relationship has been established, the researchers argue that it's time for experts to change how they approach the Zika virus. They suggest focusing research efforts elsewhere — on, for instance, vaccine development. The CDC also plans to do more studies to determine if the Zika virus causes even more developmental problems.