Mia was 48 cm long when she was born, and she tends to fill out most of her sleepers length wise, but everything is very loose around her.

My mom bought some preemie clothes by mistake and they fit her perfectly!!

I agree.. it depends on the size of the baby/the babies birth weight. Colton was almost 10lbs, and he was wearing 0-3months when he was born. He had 3 newborn outfits, which fit him, but the 0-3 month fit aswell.
His 0-3 month dont fit now, and he is in 3-6 month clothes.
You just have to go by the weight range more. Like 0-3 month clothes were like 8-12lbs and 3-6 months are 12 to like 16/18.. I cant remember.

Anna was 32cm long and weighed 6lbs 1oz at birth. She was in early baby clothes for the first 4 weeks, with some overlap to newborn (depending on where things were baught) - She then outgrew early baby but spent hardly any time in newborn because by the end of week 5 she must have had a growth spurt and while 0-3 month is still baggy on her, newborn is simply just too small...

Alex wore his newborn clothes for about the first 6 weeks, which was longer than i expected him to fit in them for as he was 9lb 1oz when he was born.

That's pretty good going! My mother's baby was 9lbs 7.5oz and never fitted in newborn clothes - went straight into 0-3 months - and it looks like it won't be long before she's in 3-6 months either! I can't believe my mother gave birth to a baby who was so much bigger than mine at the time - who was a month old!

I agree.. thats amazing! Colton was 9lbs 14oz and went straight to 0-3, already in 3-6 too.. and has been since about 5 weeks old.

I had to go out & get him newborn size clothes after we'd had him as i'd been told he would be bigger than he actually was, lol!! He doesn't seem to have been in 0-3 months that long, but has already started to wear some 3-6 month things!!

Oliver was in newborn until he was one month old- I actually remember newborn being too big for him! You wouldnt believe it now! Actually his clothes have last him for as long as the label says! ie he was in 0-3months until he was 3months!x

Don't worry too much. The age sizes are just a guide, dont rely on them too much. Go with the weight. My daughter was born at term and has always been petite. She is now 17 months old and still fits in 6-9 months clothing but moving on 9-12 now.
Dont worry, they are all different but perfect
xx

Arima was born at 6lbs 12oz and is just now
(at 3 months) getting out of new born into 0-3months ...