The dead fetus found on an American Airlines flight at LaGuardia Monday was left there by a pregnant Brooklyn teen, who suffered a miscarriage inside a lavatory as her twin sister stood guard outside that bathroom, law enforcement sources said late Tuesday.

The 18-year-old East Flatbush woman left the fetus behind on American Airlines Flight 1942 after it touched down in Queens on Monday night from Charlotte Douglas International Airport, sources said.

The flight landed and all the other passengers had deplaned when the pregnant woman rushed to the bathroom with her sister telling crew it was an “emergency,” law enforcement sources said.

A flight attendant told police the woman came out of the bathroom and looked ill, sources said.

The teen checked into Kings County Hospital at 12:30 a.m. on Tuesday and a cleaning crew made the gruesome discovery inside the Airbus A321 at about 7:30 a.m. at Hangar 5, sources said.

Police want to interview the twins, but believe this was a miscarriage and not a criminal act, law enforcement sources said. The foot-long fetus was between three to six months along, sources said.

The City Medical Examiner is working on determining how the fetus died.

“The OCME can confirm we’re investigating what we believe to be a human fetus found deceased on an airplane,” the agency said.

A rep for American Airlines said employees are “working with law enforcement on an investigation.”

The airline later said in a statement: “As we continue to learn more about this tragic and sensitive situation, we are actively cooperating with law enforcement in its investigation.”