@Caroline some people are really swollen for a few days post-op, so if you get a Macom one band size up from you it should fit for that period. The only thing to bear in mind is that when the swelling goes down, the bra will be too loose to use anymore :/
I’ve bought 2 x Macoms in my normal band size for post-op, but will also buy 1 asda post-surgery bra to tide me through the first couple of days when Im swollen. xx