I have been wondering this too! With my last 2 pgs my belly button didn't pop out, it just went flat. I didn't have my piercing then. A gf was able to leave hers in. If you wait till it gets uncomfortable (assuming it does) will it be too late to switch to a maternity bar? Also, what about c-secs?
Mine was getting too tight and was getting a bit inflamed so the lady at the piercing shop actually changed it for me (no charge except for the maternity piercing) but she did think that I wouldn't be able to leave it in the whole way through and well...she was right. My hole hasn't stretched and I can still see it so time will tell if I can put it back in again without re-piercing it! As for c/s, I would think the same as Danni cos I think you have to take all jewellry off from memory. I think I had to take earrings etc out.