Deputies said Marleni Morales Bamaca, 17, of Taylor's Bridge Road in Clinton, went to Sampson Regional Medical Center on Saturday to get treatment.
A physician determined Bamaca had recently given birth, but authorities said she denied giving birth or even having had sex.
Hospital officials called the Sampson County Sheriff's Office, and deputies went to the girl's home, Sheriff Jimmy Thornton said.
"(They) found the placenta and some other items that would indicate child birth," Thornton said, adding that a newborn boy's body was later found in a garbage bag.
"It's tough to deal with something like that, particularly a child that didn't ask to come into the world, had no control over that, and that's the sad part," Thornton said.
The sheriff declined to say how the baby died. Authorities don't know how Bamaca got to the hospital, he said.
Bamaca was released from the hospital Monday and was arrested. She was being held without bond Tuesday in the Sampson County Detention Center.
She moved to Sampson County a few weeks ago, and it was unclear who she was living with, Thornton said.