Congratulations to the most beautiful pregnant woman in the world. You're gorgeous," People quoted Hilton, as saying.
Aguilera, whose bump was visible beneath her pink chiffon mini-dress, has not confirmed her pregnancy and appeared shocked with the unexpected announcement.
As the secret was revealed, Aguilera sank her head into the shoulder of husband Jordan Bratman while in the elevated VIP booth at the club.
The couple was stunned but soon recovered and started laughing while the crowd, including Adrien Grenier, Melanie Brown, Criss Angel and cast members from 'The Hills', erupted in applause.