Maria Kang, 32, from Los Angeles, California, posted a photograph of her trim body on Facebook, with the caption: "What's your excuse?"
She appears knelt alongside her three young sons, dressed in a workout bra and matching micro shorts.
However, Maria's proud display appears to have offended several of her 80,000 followers, who accuse her of being 'insulting', 'obnoxious' and a 'fat shaming bully'.
One woman wrote: "You are . . . hurtful to women and think way too much of yourself. Disgusting is what you are!"
And a mother-of-two named Sarah added on Maria's website: "I hope you realize why some people are not just offended, but also hurt by your post as well.
"First: many of us end up with stretch marks after kids.... A LOT OF THEM! I have two kids and both have caused my skin to stretch soo bad my stomach, from my rib cage straight down the the middle of my vaginal area, look like the U.S. road atlas.
"Second: I am an avid runner. I run my two kids in a double jogger. Do I look like a runner? Nope. As a matter of fact, unless people know me, they don't believe me. I do half and full marathons.
"People like you who post pictures like this make people like me cry because I know that without surgery to lose the extra skin I will never look like you.
"So what's my excuse? I do not have the same great genetics you do."
However, Maria, who runs an fitness-focused non-profit organisation from home, said she didn't intend to upset people and she hoped the photo would inspire others to feel better about themselves.
She wrote on Facebook: "I'm sorry you took an image and resonated with it in such a negative way.
"I won't go into details that I struggled with my genetics, had an eating disorder, work full time owning two business', have no nanny, am not naturally skinny and do not work as a personal trainer.
"I won't even mention how I didn't give into cravings for ice cream, french fries or chocolate while pregnant or use my growing belly as an excuse to be inactive.
"What I WILL say is this. What you interpret is not MY fault. It's Yours. The first step in owning your life, your body and your destiny is to OWN the thoughts that come out of your own head.
"Maybe it's time we stop tip-toeing around people's feelings and get to the point."
She said she does have stretch marks and excess skin from three pregnancies but these 'scars' encourage her to 'keep striving' and 'never stop'.
"Motherhood is beautiful AND it can make you better!" she wrote.
She said she likes to do 30 to 60 minutes of strength training and cardio every morning, five to six days a week. On a typical day she tries to get to the gym by 6.30am and then returns home to 'feed, clean and play' with her sons.
She then focuses attention on the non-profit business she runs, Fitness without Borders, which promotes fitness education in schools and local communities.
"I wanted to inspire people. I wanted to say, 'I know you think you don't have time if you have kids. But if I can do it, you can do it, too'," she said.
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