The girl was taken to the hospital by her teachers where doctors discovered she was in the later stages of labor, reports Metro.
The unnamed schoolgirl from southeast Brazil was found to be seven months pregnant.
The baby was delivered safely last week.
Officials fear she became pregnant after possibly being raped by her 40-year-old stepfather.
He has been accused of child rape and illegal possession of firearms.
The girl’s mother said she was unaware the child was pregnant and had not noticed her daughter’s stomach was larger than normal.
Metro reported police officers were told by the girl that the stepfather had threatened to kill the mother and a brother if the young girl ever revealed the abuse.