She arrived onstage in a purple blazer and black trousers by D&G, and said: "I want you to feel the love that's growing inside of me."
Afterwards, the delighted singer pulled open the jacket and rubbed her bump affectionately.
The camera then cut to the delighted dad-to-be. He smiled widely at the camera while Kanyne West clapped him on the back and shook him.
Beyonce's news eclipsed the night's other events.
Lady Gaga's much-hyped opening number – during which she performed as her greasy, leather-jacketed male alter-ego Joe Calderone – took a back seat.
So did the evening's meticulously planned wild moments, from Nicki Minaj's origami-like outfit to a dance-off between the members of Odd Future and Jack Black, Will Ferrell and Seth Rogen.
Katy Perry, who had the most nominations coming into the show with ten, won three trophies, including video of the year for the inspirational clip Firework.
"I feel like I'm doing something right when I sing that song," said pink-haired Katy, who arrived in a quirky Oriental-themed cut-out dress.
Britney Spears captured the night's first award, for best pop video, and later was honoured with an MTV Video Vanguard award for her visual legacy.
Adele provided another of the night's highlights, delivering a powerfully understated performance of Someone Like You, from her top-selling album, 21.
She came away with three trophies, tying with Russell Brand's wife.
Crazy in love: Beyonce's intensively private relationship with Jay-Z
It's a little princess!