How do I know when to size up in diapers? Lucy is 12 weeks old, just over 12lbs, and wearing pampers swaddlers size 1s. We're almost through this case, and I have a case of 2s, but they seem so big! We have minor pee leakage issues, but only if she's been in the diaper a long time, and the occasional poo blowout up the back, but not out the legs.