Hey,
A 34A will have the same band size as a 34C. As with women’s clothes, lingerie also varies in fit from retailer to retailer and sometimes within the same retailer. I wouldn’t get yourself confused and would just wait until your post op to get measured to find out what size bra you should be wearing.
If your trying to get an idea on size then loads of ladies recommend doing rice sizers – where you measure out your implant size in grams stuff it into an old pair of tights and put them in your sports bra.
Xx