[EllaC]

Actually if her underbust is 30, then at most she should be wearing a 32 band, which likely means she's a 32C

Depends on how you like your fit. My underbust is 29 and 32 bands can't even close around my back and the ones that do are sooooo tight I can't breathe. If you like that tight strangulation feel then all good.
But in all seriousness countries differ and all bra calculators differ and manufacturers differ. For the most part I measure 30 under loosely and 29 snug. I find all 32's too tight . But yeh it's up to the individual and how they like their fit. In my country I think we go by the +4" theory and its bang on for me.

Oh. Yeah, it can be confusing.

If her bra isn't moving around on her then she's probably right on.

Hello! Thank you both for your replies!

You may be right about the bra size, I have no idea. The 34B is just what fits me best at the moment. I figured I'd wait a couple months, see if I get any growth, and then go for a professional fitting when I need new bras! *fingers crossed*

Well actually the best thing you could do is go and fitted NOW while you're starting nbe. That way a professional fitter can let you know if the 34 is too loose OR the 32 is too tight. By wearing the wrong size you can compromise growth. Ie I just "assumed" I was the smallest size cause I never got fitted. Turns out I was bigger than I thought. See if I use an American bra calculator it puts me wearing 32d bras which kinda works but too tight but if I use the Australian/NZ bra calculators I should be a 34 band.

Too tight meAns poor circulation, too loose means no support. I'd do it now and I'm curious for too to see what they say cause I'm pretty much same dimensions as you.