Authorities say she gave birth in a bathroom stall at the Cumberland County Fairgounds during a Cinco de Mayo celebration on May 4, 2008. The girl was 14 at the time.
Cumberland County Prosecutor Ronald Casella initially sought to have the Bridgeton teen, who was already the mother of one child, tried as an adult.
But Casella reconsidered, saying the girl may have been a victim, too.
Authorities say they are seeking the newborn's father.
Sanchez's sentence is up to a judge.
Information from: The Press of Atlantic City