Kristin Cavallari
is working on her fitness ... with a baby bump in tow!

The former "Hills" starlet shared a snapshot via Instagram over the weekend, showing off her blossoming belly at the gym.

"Gym selfie ... baby bump getting big," the blonde beauty wrote along with the cute pic.

The expecting mom looked ready for her workout in a tight black T-shirt, black leggings and bright orange and white sneakers.

Since debuting her baby belly in November, Kristin has stayed mum on whether son Camden, 17 months, will be joined by a little brother or sister.

"It's harder because I'm tired, and then I still have to be a mom to Camden," she told People magazine of her pregnancy. "But it is easier in the sense that I know what to expect."

We can't wait to see the little bundle of joy!

Click "Launch Gallery" above to see more expecting celebrities.

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