On Saturday, Simpson shared a behind-the-scenes photo of her upcoming ad. Wearing a form-fitting black dress, Simpson stands behind her chair.
"Too proud to sit down!!" she captioned the image.
Simpson told USA Today last December that two years of pregnancy "took a toll" on her body, but said she started trying to lose weight soon after Ace was born.
"People are going to talk about this no matter what," she said of media scrutiny about her weight. "I might as well choose to live a healthy lifestyle."
"It does get old having people harping on how much you weigh or your body after pregnancy," she added. "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."