"Congratulations to the most beautiful pregnant woman in the world. You're gorgeous," Hilton announced at about 2 a.m. Sunday at an Aguilera-hosted party at LAX Nightclub in the Luxor.
Aguilera, who has never confirmed being pregnant despite a visible bump beneath her pink chiffon mini-dress, appeared temporarily shocked as she sat in her perch in the elevated VIP booth. She sank her head into the shoulder of husband Jordan Bratman as her friends looked stunned.
But she quickly recovered, and the two looked at each other and laughed as the crowd – which included Adrien Grenier, Melanie Brown, Criss Angel and cast members from The Hills – erupted in applause. (Aguilera's rep declined to comment).
Hilton's surprise announcement, coming after she earlier put her hand on Aguilera's stomach and the pair giggled, was the highlight of the LAX party, one of many rocking Las Vegas on the weekend of the MTV Video Music Awards.
Here's how the stars are partying ...
Kanye West took to the stage at the Hard Rock after 2 a.m. Sunday, performing an eight-song-set at the Rolling Stone bash. Backed by an all-female orchestra of 12, West, in a white blazer, black shirt and jeans, tore through his standards before being joined on stage by an Jamie Foxx, who beat-boxed into a version of "Gold Digger" that had West smiling as the two rapped at one another.
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Mel B and hubby Stephen Belafonte chatted with comedian Sarah Silverman at the Maxim/Dockers party at Mandalay Bay's Moorea Pool on Saturday. At the bash, Silverman told PEOPLE exactly why she loves Vegas: "I'll be honest, I like 'hotel life,'" she explained. "I got in. I shut all the windows. I took a nap. I woke up. I watched Superman Returns on TV." Also at the bash: soul singer Joss Stone, whose audience included Carson Daly, Shar Jackson and Entourage stars Grenier and Kevin Connolly. Ryan Cabrera also made a splash at the bash – literally. He fell into the water by the entrance!
Hilton, Angel and Ciara and other celebrities partied as 50 Cent performed songs off his new album, Curtis, (plus oldies) by the pool at Hard Rock. Afterward, everyone headed to Body English for Absolut 100's 50 Cent album release party. (Click here to get the scoop on Britney Spears's night there.)
Maroon 5's Adam Levine took to the stage after midnight on Friday in the casino area of the Hard Rock. Why? A cover band was playing Maroon 5's "Harder to Breathe" – so the singer got up from one of the tables (where he was gambling) and sang with them! Earlier in the night, he dined at Nobu.
Reported By JENNIFER GARCIA, REAGAN ALEXANDER, JESSICA HERNDON, MARISA LAUDADIO and MARK GRAY
Click here for more sightings of Nelly Furtado, Eve and other VMA stars!