"That's what she said," Fallon joked.
"Ohhhh! I was trying to keep it private, but we're having a boy!" blurted out Larter, who's expecting her first child with her husband, writer-actor Hayes MacArthur.
Larter said pregnancy has been an "amazing ... exciting time."
Fallon, fresh off some rave reviews for hosting the Emmys, then asked the Resident Evil: Afterlife actress about being from New Jersey (Cherry Hill, outside of Philadelphia) and how Real Housewives of New Jersey and Jersey Shore are or aren't representing the state well.
"It gives Jersey such a bad rap ... even the people who are on Jersey Shore, they're not from Jersey, they're from New York," she said, admitting that the show is a "guilty pleasure" though.