Not a maternity bra expert either, I would definitely recommend a trip to Bravissimo. IIRC, maternity bras tend to have a bucketload of clips so although you will possibly still get a little broader round the ribs between now and birth your back measurement will go back down fairly quickly. Based on your measurements just now I'd agree with Sorrel.
TBH, I think it depends quite a lot on how your boobs behave once your milk comes in. I bought a maternity bra to wear occasionally at the end of my pregnancy (I stayed in underwires up until the end unless I was lazing around at home) which fitted fine. However it was miles too small once my milk arrived, as in several cup sizes too small. I think it's kind of hard to predict as some women grow a lot and others only a little. So if you have some that will tide you over it might be worth waiting until afterwards to buy the longer-term supportive ones.
I have twice been in Bravissimo and seen women with tiny babies come in, one of them was in tears she was so uncomfortable. The shop was mega busy but two of the assistants pretty much dropped everything and took her in to a back room (I think it might have been a staff room as the fitting rooms had a queue) with one fitting and one holding the baby!
Thank you for the advice. Lastprime I had a baby I remember waking up a few days after birth with boobs that resembled dolly Parton. I guess the same will probably happen this time. By the sounds of it I will need to just wait til baby is here. I was hoping I could buy one or two to see me through but not sure if it's worth it if they will be useless when milk arrives.
I will probably be back looking for help once baby has vacated my tummy!
I was one of those on Bravissimo with a tiny baby, and agree that their nursing bra range is small, I could only get a royce bra and it was dire. I ended up buying clips and converting some bras myself. There have been a few mentioned on here more recently, I'm sure someone will come along to tell you which brands are good.
I tried out loads - M&S mostly following dc1, emma jane ones by the time i had dc2 - liked the stretchyness compared to the then very stiff M&S ones. I found bravissimo following third dc and LOVED this one it's so much more supportive and pretty too. Noticed a bigger range in M&S the other day that looked much better than when i last looked.