Cavallari revealed this weekend that she is expecting her first child with fiance Jay Cutler, who she became engaged to for a second time in November.
Montag wrote on her Twitter account: "Congrats on your life changing news!!!! You are going to be such an amazing mommy!! Sending you So much love!! Xoxo (sic)"
Patridge wrote on her own feed: "Congrats Kristin!!!:) gonna be such a cute baby xoxo."
Former Laguna Beach star Cavallari first became engaged to Cutler in April 2011, although the couple split a few months later, before reuniting and becoming engaged once again at the end of the year.
She recently said: "Sometimes, in order for things to get better, they have to end - even if it's momentarily, and that was the case with us. We didn't break up because of love lost, We loved each other just as much as we always did when we weren't together."