I always go up a size early, because DS has chunky thighs. We use huggies, so I went up to the infant size after a few weeks (DS was 4.64kg at birth), and if I known the inbetween size existed, be probably would have started in that.

At his weigh-in a month ago he was 12.1kg, and he is currently in 13-18kg nappies, and sometimes even they seem too small.