"Gym selfie," wrote the 27-year-old, who is expecting her second child with husband Jay Cutler. "Baby bump getting big!"
The reality star-turned-designer can be seen wearing a form-fitting black T-shirt, leggings and colourful trainers.
Kristin announced her pregnancy in October, but so far hasn't revealed whether her 17-month-old son Camden will be joined by a little brother or sister.
"It's harder because I'm tired, and then I still have to be a mum to Camden," she told People of her second pregnancy. "But it is easier in the sense that I know what to expect."
It has been a big month for Kristin; she recently celebrated her 27th birthday with her husband Jay, and shared a sweet photo from their evening out together.
"Thank u for all the birthday love. Enjoying a 'mock-tail' for the big 2-7," she captioned an Instagram photo of the couple toasting her special day.
A few weeks ago, the Illinois-based family also discovered that they were staying put for the foreseeable future after quarterback Jay signed a seven-year contract with the Chicago Bears.
"Congratulations to my hubby!!! 7 more years in Chicago… we couldn't be more excited," Kristin wrote on Instagram.