The pope unequivocally rejected abortion as a response to the crisis in remarks Wednesday as he flew home after a five-day trip to Mexico.
But he drew a parallel to a decision by Pope Paul VI in the 1960s to approve giving nuns in Belgian Congo artificial contraception to prevent pregnancies because they were being systematically raped.
Abortion "is an evil in and of itself, but it is not a religious evil at its root, no? It's a human evil," Francis told reporters. "On the other hand, avoiding pregnancy is not an absolute evil. In certain cases, as in this one (Zika), such as the one I mentioned of Blessed Paul VI, it was clear."
Francis was responding to a reporter's question about whether abortion or birth control could be considered a "lesser evil" when confronting the Zika crisis in Brazil, where there has been a spike in babies born with abnormally small heads to Zika-infected mothers.