I ordered these for my 2 month old grandson who is 12 lbs. and  just outgrowing his Carter's 0-3 months onesies. Logically he should move into the 3-6 month size, but after reading the reviews here, I ordered the 6-9 months size. These 6-9 months Gerbers - after laundering as instructed - are BARELY any bigger than his Carters 0-3 months. They may last us a couple weeks if we're really lucky.