Zika virus was found in the brain of a fetus, the strongest evidence yet that the virus causes abnormally small heads and incomplete brain development, the Verge reports citing an article in The New England Journal of Medicine.

This is the first documented case of virus transmission from mother to child, though it is not a definitive link between Zika and fetal abnormalities — a connection first suggested by the Brazilian minister of health.

The Zika virus appeared to particularly favor neurons, the report found. Damage from the virus may have halted brain development at 20 weeks, though it's not clear how the virus was infecting nerve cells. Some structures found at autopsy showed that the virus was reproducing in the fetus's brain.

Zika has spread to many countries in the Americas, though Brazil bears the heaviest caseload, with estimates of 440,000 to 1.3 million infected as of December. The number of newborns with microcephaly has increased twentyfold in the northern part of Brazil, the health minister has said. That led to the suspected link — and five public health agencies in multiple countries have advised women not to get pregnant as a result. Until now, though, there was no evidence of virus transmission from a pregnant woman to her fetus.

"This is the critical point: you have a mother who's infected, a fetus that's abnormal, and in the fetus, you have the genetic signature of the virus," says Andrew Pekosz, director of the Center for Emerging Viruses and Infectious Diseases at Johns Hopkins Bloomberg School of Public Health. "This is clear data showing Zika can infect the fetus." This case report isn't proof that Zika is causing microcephaly, but it makes the link much likelier, Pekosz says.