gah maternity boobs are so frustrating.
I seem to go up a back size about the time I get a BFP.
Mine didn't grow in size at all last time, but this time...hmmm. Up from 12D to 14D already at 18 weeks.

If you are going to get maternity bras I got great advice when being fitted the first time. Make sure you have at least 3 hooks (or the number of mnths left if less) and a bit of room in the top.

Target make quite a range of wirefree bras for quite cheap too.

tears in the change room are totally acceptable.

ETA I've splashed out (well DH has) on some Cake Lingerie bras this time round. They have a flexi underwire that sits off the breast, and they are seriously comfie. Even though I've spent the better part of 6 years in wire free bras, there's nothing like good support for bigger boobs.