Dwayne 'The Rock' Johnson and Pregnant GF Debut Baby Bump

Dwayne Johnson is celebrating a huge debut, and it ain't on a movie screen ... it's in his longtime girlfriend's womb.

The Rock and Lauren Hashian proudly showed off her baby bump in Hollywood Monday night at the 'Jumanji' premiere event. The photo op came just a few hours after they announced they're expecting their 2nd child together. The couple's first -- 2-year-old Jasmine Lia -- helped reveal they're having a girl. Again.

Dwayne also has an older daughter from his previous relationship.