Authorities are hunting for the baby's 17-year-old father after the girl fell pregnant aged just eight and gave birth last week.
The baby was born on January 27 in Mexico's western Jalisco state.
"We are looking for the young man to get his story because she does not understand what has happened. This is a rape or child sex abuse case," Jorge Villasenor, from the state prosecutor's office, was quoted as saying.
The girl and her baby were discharged from hospital and are believed to be doing well.
However, the hospital said it would have to do extensive follow-up due to the mother's age.