After she was taken to a hospital, she gave birth to a baby and named him Francis after the current pope. The nun, originally from El Salvador, claims she had no idea she was pregnant.
The sister belongs to a convent which is located near the city of Rieti, which has a population of 47,700.
The nun belongs to the "Little Disciples of Jesus'' convent in Campomoro near Rieti, which manages an old people's home.
As news of the nun's pregnancy has spread, the mayor of Rieti, Simone Petrangi, asked local residents and media to give the woman privacy.
Clothes and donations have been collected and sent to the hospital where she gave birth.
"I did not know I was pregnant. I only felt a stomach pain," she told the Ansa news agency.
Other nuns at the convent also expressed shock at the mysterious pregnancy of a holy sister at their order, saying they were "very surprised", according to Italian media reports.
Don Fabrizio Borrelio, a local pastor, says he believes that the nun is telling the truth about being unaware of her pregnancy. He said the nun plans to take care of the baby herself.
Why am I not shocked or surprised? Let me leave that story for another day.