Isla Fisher didn’t shed her pregnancy pounds through a regimen of sweat and starvation.

Ask the Aussie actress about her trim figure since giving birth to daughter Olive in October, and she grins mischievously.

She lifts up her filmy, flowered blouse and crows the magic word: “Spanx!”

“You know them? It’s night and day!” the “Definitely, Maybe” star says of her tummy-smoothing undergarments.

Since Fisher is still nursing, calorie-counting isn’t a priority as she digs into a jar of M&Ms.

“I really felt like there’s so much pressure on young women in show business,” she says. “There’s so much body fascism. I don’t buy into any of it. But I do have good genes.”

For all her sparkly sociability, Fisher reveals nothing about her relationship with her fiance, comedian Sacha Baron Cohen (“Borat”), or their life together as parents of Olive.

“Motherhood is my favorite topic in my personal life, but I never talk about it publicly only because I don’t want to draw any attention to her,” says Fisher, 32. “I want to protect her.”

Fisher won’t even talk about balancing movie offers with her role as a new mom.

“I can’t answer without touching on the most important thing in my life,” she says. “So, I’m scared to. I’m sorry.”

She will say that work right now “isn’t as big a priority.”

It certainly isn’t for April, the sharp-tongued sprite Fisher plays in “Definitely, Maybe.”

She’s the kooky, cute pal of co-styar Ryan Reynolds who just might be his Miss Right. Other contenders for the title are Rachel Weisz and Elizabeth Banks.

“I think it’s told from a male perspective, so hopefully guys will go,” Fisher says. “It’s not just one girl in the movie. It’s three. It’s a romantic whodunit.”

To Fisher, April represents a younger version of herself.

“April was very similar to me when I was in my early 20s. She’s free-spirited and apolitical, doesn’t believe in marriage and is desperate to travel the world,” she says.

“She has this arc and ends up working for Amnesty International, which I totally take my hat off to. I wouldn’t have the strength to work somewhere like that. I’d be crying every day at every crime against humanity that I knew about.”

The redhead most famous for taming Vince Vaughn‘s lothario in “Wedding Crashers” says she’s getting pickier about what roles she takes.

Since cannon-balling to fame in that 2005 comedy, she has done dark drama (the positively reviewed but little-seen “The Lookout”) and goofy comedy (last year’s silly romp “Hot Rod,” starring Andy Samberg).

“I’m choosy, definitely,” Fisher says. “I’ve made a lot of product in my life that I’m not proud of. I’ve had to work.

“Now that I’m fortunate enough to get to choose, I’m fussy. Now, I don’t have to make decisions as much from fear. It’s given me freedom.”

The birthday bunch

Actor George Kennedy is 83. Author Toni Morrison is 77. Director Milos Forman is 76. Singer/artist Yoko Ono is 75. Actress Cybill Shepherd is 58. Actor John Travolta is 54. “Wheel of Fortune” hostess Vanna White is 51. Actor Matt Dillon is 44. Rapper Dr. Dre is 43. Actress Molly Ringwald is 40. Actor Tyrone Burton (“The Parent ‘Hood”) is 29.