One of the reasons everyone likes macom is because the cups expand so you you don’t have to guess the cup size. I’ve got a two large Macoms and I’m a 34/36 back so I think you may be okay with a medium one if your back is smaller. You could put the macom one on and get the second bra once you’ve had the op. You could also guess the size of the cup from what you’ve told the surgeon you expect to be after the op but obviously that’s not guaranteed you’ll turn out that size afterwards xx