The 24-year-old singer - who gave birth to Bronx Mowgli, her first child with husband Pete Wentz, in November - is determined to ditch her baby weight, and so has embarked on a "mommy and me" workout.
Ashlee - who has already lost half of the 30 lbs she is aiming to shed - is following Jackie Keller's 30-day Body After Baby plan, meaning she is completing low-impact exercise such as playing peek-a-boo with Bronx.
The star is also watching what she eats in an effort to shift the excess pounds, and has been feasting on Nutrifit meals and snacks which she has delivered daily.
A source explained: "Ashlee is swapping French toast and sugary syrup for multi-grain bread and berries."
She has also cut down on her fast-food intake, choosing to eat just one slice of pizza piled high with vegetables when Pete brings home the calorific treat.
Ashlee recently leapt to the defense of her sister Jessica, who has been subjected to scathing comments after she was pictured with a seemingly fuller figure.
Ashlee wrote on her blog: "I am completely disgusted. Since when did a woman's weight become newsworthy? How can we expect teenage girls to love and respect themselves in an environment where we criticize a size two figure?"
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