"Being a mom is the most incredible feeling," she tweeted Saturday. "I am so in love with this little boy!"
The reality star gave birth to her son, Camden Jack Cutler, Wednesday.
Cavallari, 25, is engaged to Chicago Bears quarterback Jay Cutler, 29. The couple announced their engagement and pregnancy in January following a brief split, People magazine reported Saturday.
Cavallari is best known for her work on TV's "Dancing with the Stars" and "Laguna Beach."