My little bubba was only a little guy and was in mostly 0000 at first. I think I had a few 00000?? It's hard to remember without going and checking. I had to actually go and buy some cos I'd worked on 000 as well...lol. So it might be handy to just buy a couple of 0000 things and more after if you need them. He's now outgrown 000 and into 00 and he's just shy of 5 months. I've been buying size 0 now for him to grow in to. One thing I will say though is it does depend on the makes to the sizes which fit bubs!! With bonds wondersuits I found (and most ppl I've spoken to say the same thing) that you need the next size up from what they're in ... so if bubs is in 000 in most other brands then the 00 in the wondersuits.