DS1 was 7lb2oz, 48cm and wore 0000's for approximately 4 weeks if i remember correctly.
DS2 was 8lb2oz, 52cm and he only wore 0000's for about 2 weeks
DS3 was 9lb15oz, 59cm long so he never got into 0000's at all ....he was too big all round (weight/length).

Maybe buy a few items but leave the tags on that way if bubs does turn out to be a bit bigger then your expecting you can return them and get more in a size that fits. Goodluck not long now until your little one will be making her entrance