When I was prg with my DD I just left my normal one in (it's a banana bar with a ball on etiher end). The midwives would ask me 'when are you taking that out' and I'd say 'if and when it gets uncomfortable'. It never did so I just left it and it was fine, I never took it out.

I think for those people who's belly buttons don't 'pop' (mine didn't, stayed exactly the same) it's not an issue.