We used Newborn nappies until Claire was about 3 - 3.5 months - I think she was around 5kg by then. We only started using Infant size in the last few weeks. We went up a size because she had started leaking out of the newborn nappies quite often, although they did still fit her little bum just fine. The bigger sizes have more absorbancy as well as just being bigger and your baby might need that extra absorbancy before they are out the the weight range. The weights on the packaging are just guidelines to give you a rough idea.