Houston homicide investigators interviewed both the girl and a 14-year-old boy believed to be the father.
The girl, who authorities have not identified, told police she didn't know she was pregnant. Preliminary autopsy results indicated the fetus was stillborn and not viable, police said.
The Boeing 737 plane landed at Bush Intercontinental Airport on a non-stop flight from New York's LaGuardia Airport. The plane, which was about 30 minutes late, carried 144 passengers and six crew members.
A cleaning crew found the fetus inside a wastebasket in the bathroom of a Continental Airlines plane about 15 minutes after the plane landed.