The 30-year-old designer shared a photo with her 30-year-old fellow The Hills alum Wednesday ahead of the birth of Montag's first child.
"This girl. Love her and that cute little belly. Thanks for coming by @heidipratt," she captioned the picture on Instagram.
The snapshot shows Montag cradling her baby bump as she and Cavallari smile for the camera. Montag later posted the same photo on her own account.
"So much fun catching up with @kristincavallari," she wrote.
"Happy birthday to the love of my life!" the star wrote. "You are the best husband and I know you will be the most amazing father!" "We are all so blessed to have you! Thank you for brightening up everyday of my life! I love you beyond words!"
Cavallari herself shares 5-year-old son Camden, 3-year-old son Jax and 21-month-old daughter Saylor with husband Jay Cutler. She opened up about Saylor in an interview with People published Wednesday.
"We call her the honey badger because she's just pure sass," the star said. "She's adorable, but she will literally rip your face off."
Cavallari and Montag appeared together on the MTV series Laguna Beach before starring on The Hills. Fellow Hills alums Lauren Conrad, Audrina Patridge and Whitney Port have also welcomed children since the show ended in 2010.