"I started working out when he was 4-weeks-old," Cavallari, who welcomed baby Camden with fiance Jay Cutler in August 2012, told Us. "Breastfeeding burns 500 calories a day, so that's been very helpful." Cavallari also hit the gym to jog, but didn't go overboard. "[I've been] taking it easy and not killing myself. I haven't really been stressing about it either. I think that helps."