Credit: Jeff Kravitz/FilmMagic

Mariah Carey -- who is expecting her first child with husband of two years Nick Cannon -- will be a "great" mother, her Precious costar Gabourey Sidibe tells

"I have faith that she will be," she told Us at a Fresh Air Fund gala in NYC Thursday. "She's pretty loving, and she's very nice."

Sidibe, 26, said she wasn't surprised when she heard the pregnancy rumors.

"As soon as anyone gets married, everyone's like, 'Have babies!'" she told Us. "I'm like that. As soon as my friends get married, I'm like, 'When are you having babies? Have babies already!'"

A source confirms in the new Us Weekly (on newsstands now) that Carey, 41, is "in the early stages of pregnancy," but is waiting until after the first trimester to announce it publicly.

"If & when my wife is ready to make ANY announcements about private matters she will do it personally," husband Cannon, 29, Twittered Wednesday.

But he said they've already picked out baby names.

"We're gonna do it like George Foreman," he joked on his morning radio show earlier this week. "We're gonna name all of our kids Mariah no matter if they're boys or girls. ... Mariah No. 1, Boy Mariah, Man Mariah, Tall Mariah. It's gonna be a house full of people named Mariah."