My little one was born in April and at 8lb 5oz (3.5kg ish) initially wore both 0000 and 000.

At 8 months and almost 8kg she is wearing mainly 00 but can also wear 0 and some 000 still fit (not the all in ones though cos she is too long).

She has some gorgeous sleeveless outfits but because it has been a cool Summer so far hasn't worn many of them. i reckon you don't buy too far in advance cos what you need might be different to what yo think you will need and you might find you get presents or second hand clothes that bub can wear.

i don't know about the definitions so it is interesting to hear what everyone is calling them.