updated 05/31/2012 AT 11:15 AM ET
•originally published 05/31/2012 AT 8:00 AM ET
“I think anybody who’s gone through a pregnancy, after they have the baby, it’s like, ‘I need to do something about this,’ ” she tells PEOPLE, in its latest cover story. “For me, I really want to do something that is a lifestyle, because in the past, I’ve been known to yo-yo diet.”
So, Simpson, who Tweeted “so excited to be part of the Weight Watchers family” Wednesday, signed on to work with Weight Watchers, a program she’d planned to do before she learned she was pregnant. When she got the good news, she stopped counting her points.