After giving birth to her second child, son Ace Knute, last summer, Simpson dropped a whopping 70 pounds.
Wearing a skin-tight white dress in the ad, the 33-year-old singer and fashion mogul says she's "never been happier."
"I have a newfound determination that I'm really proud of."
Simpson said last December that pregnancy "took a toll" on her body, and said she expected the media scrutiny that comes with pregnancy weight gain.
"It does get old having people harping on how much you weigh or your body after pregnancy," she said.. "I need to feel comfortable in my own skin. People are going to pick me apart no matter what I do. I'm always going to have critics out there who will make a story out of something."
[E! News, NY Daily News]