It was a great, fun night at LAX in Las Vegas Saturday ... Christina Aguilera hosted the party, the guest list was sparkling with stars like Paris Hilton, Adrien Grenier, Melanie Brown, Criss Angel & Xtina's hubby Jordan Bratman.

But Xtina & Jordan got the surprise of their lives when at 2 am, Paris Hilton took to the mic & innocently overwhelmed Xtina with a compliment ... but at the same time exposing a secret that Xtina hasn't announced officially!

Paris announced: "Congratulations to the most beautiful pregnant woman in the world. You're gorgeous!" ooops!

Christina & Jordan looked shocked at that point! She leaned on her hubby's shoulder as their friends at the VIP booth looked stunned!
But they soon got over it & continued to part-ay!

Christina tried to hide her visible bump beneath a pink chiffon mini-dress, but it was pretty obvious!