It depends on length rather than weight. DS was long so he grew out of things quickly from shoulder to crutch (bodysuits), he wasn't in 0000 long at all maybe a week or two. Then 000 only maybe 2mths max? Try not to spend much on little stuff as they do grow out of it and buy things that are practical. Overalls & dresses look cute but so unneccesscary on a newborn. Bodysuits are so much easier and comfortable for them.
DS - Nov 2008 & DD - Feb 2012