The 25-year-old Cavallari wrote Wednesday on Twitter that she was "thrilled" to announce the baby's birth that morning. He weighed 7 lbs., 9 oz. She added that everyone was "doing great."
Her publicist Jack Ketsoyan confirmed the birth.
Cavallari and the 29-year-old Cutler announced their engagement in 2011 but broke up briefly last summer. They reconciled a few months later.