Well, I'm going to assume that SilverDaughter isn't going to be pregnant anytime soon.

Again - piercings are removable. When I had my navel piercing, I knew that I might have to remove it at any time for a lot of reasons... pregnancy, laparoscopic surgery, emergency appendectomy, etc. etc. Same goes for the nipple and other girly piercings. Those will most likely get retired if I ever choose to bear children someday. At that point I'll just think to myself "well that was fun while it lasted".

Another key point about navel suitability - it's not as tied to thinness/fatness as another posted suggested. While that can be a factor, the more important determining factor is the existence of a well-defined lip of skin overhanging the rim of the navel. Piercings through parallel tissue (like earlobes, nostrils, etc) have a good chance of succeeding. Piercings through flatter skin areas like a shallow navel or eyebrow piercing tend to migrate and ultimately reject.