Hi Jess
i bought 0000 and 000 for my first son and he was born at 6lb8ozs and needed 00000 but my second son was 5lb2ozs and needed 000000 for about a month and needed 00000 for 3 months and now he wears 00 in nearly all outfits and he is nearly one! It depends on the size of the baby and the way he/she develops.
Good luck and will keep tabs on your progress.