Personally, I like a front-snap (or front aplix) diaper for big waists. Then you can fasten them under the bulk of the belly without having a giant poof like you get with a side snap diaper on a big waist. A good diaper with tons of adjustability would be CuddleBuns. If the baby is at 12 pounds now, these should fit and since they are Single Sized, they'll fit for a looonnnnggggg time to come. At 12 pounds, she'd probably use them with the front panel folded down. Still lots of adjustability.
Hope this doesn't sound too spammy since I make CuddleBuns, but I'm not even open right now so I promise this is just an honest recommendation.
