It was for me. Mine was in 0-3 mths for ages as he didn;t gain weight well so i bought loads of 3-6 mths and then he suddenly caught up with himself and whizzed through 3=6 mths so I was very sparing with his 6-9 mths and he's been in them since August - he;s just getting into the 9-12 mths now -I;ve bought loads of 9-12 mths as I was getting bored of the 6-9 mth clothes - for me there was more than justa bit of difference b etween 6-9 and 9-12