"Even when it felt impossible, I chose to work harder."

Credit: Kevin Mazur/Getty Images

Jessica Simpson took to Instagram Tuesday to announce that she's lost 100 pounds in six months, after giving birth to her daughter Birdie Mae in March.

The 39-year-old former pop star (and current businesswoman) shared the news along with two photos of herself in a black dress cinched at the waist with a belt. One of the images shows her holding her holding her Birdie Mae. "6 months. 100 pounds down (Yes, I tipped the scales at 240 )," Jessica wrote in the caption. "My first trip away from #BIRDIEMAE and emotional for many reasons, but so proud to feel like myself again. Even when it felt impossible, I chose to work harder. "

Clearly, dropping 100 pounds in six months sounds pretty remarkable—and Jessica's followers chimed in to ask what exactly she did to lose the weight (seriously–most of the comments are asking her how she did it). Jessica didn't divulge how she lost the weight (or at what rate), but it begs the question: Is it healthy to lose that much weight in such a short amount of time?

The short, totally underwhelming answer: It depends.

Take Jessica's recent pregnancy, for instance: She mentioned her highest weight was 240, which means that some of those pounds that she eventually lost would have been the baby, placenta, fluids, and blood volume, says Cynthia Sass, Health contributing nutrition editor.

Whether or not Jessica breastfed could've also played a role in her weight loss. Breastfeeding burns extra calories (about 300 to 500 a day, according to the U.S. Department of Health and Human Services’ Office on Women’s Health), which helps speed up weight loss in postpartum women.

According to the US National Library of Medicine, a division of the National Institutes of Health, it's healthy for postpartum women to lose about a pound and a half a week, and most women return to their pre-pregnancy weight by six to 12 months after delivery.

Postpartum weight loss aside, healthy weight loss in general takes place gradually, typically at a rate of one to two pounds per week, according to the Centers for Disease Control and Prevention—that's because, per the CDC, healthy weight loss isn't about a diet or a program, it's about lifestyle changes that can last long-term through healthy eating and exercise habits.

Jessica opened up about changing her fitness and food routine to lose her pregnancy weight in a July interview with People. "I am working really hard right now," she said. "It's not easy at all, but I am determined to feel good. I have been doing a lot of walking—getting my steps in not only burns calories but it also helps me clear my head and get focused."

She also said she made changes to her diet. "I am eating healthy too—I discovered I really like cauliflower. Who knew it could be a substitute for almost anything?!" she said.

So, while there's not enough information about how Jessica lost weight to determine whether it was entirely healthy or not, it still serves as a reminder that everyone loses weight differently (especially after giving birth) and that there are a lot of other factors that come ito play when it comes to weight loss.