With my first I had a REALLY deep belly button... like really deep... the dude who pierced my belly said I had the weirdest belly button he'd ever seen *blushes* lol So it never even got close to popping! But then also never went back to being as deep.

With DS2 I thought it was going to pop as it started heading towards being slightly flatter at about 28ish weeks from memory... I went 5 days early, and it didn't really move from 28 weeks lol. It wasn't completely flat, but wasn't far off! :P