Nicole Richie may be living life to the fullest since the Jan. 11 birth of baby daughter Harlow Winter, but you would never know by looking at her post-baby body.

The 26-year-old first-time mom has been showing off her slim but healthy physique, most recently a Grammys afterparty and at the opening of the Broad Contemporary Art Museum on Feb. 9, where the Simple Life star looked so svelte it’s hard to believe she gave birth only weeks ago.

Now OK! can reveal the star’s slimming secret isn’t some miracle diet or fitness routine, but waist-cinching undergarment Spanx!

"She sometimes wears two pairs at a time," a pal of Nicole reveals to OK!. And while the new mom is keeping this secret from the public, her fiancé Joel Madden, 28, is all too familiar with them. "He’ll help her find them in her closet if they’re running late," says the friend. "But he thinks she looks great just the way she is."

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