Like an earlier post said, you are the best judge. Bottom line, some people's skin isn't able to tolerate even flexible maternity hardware, while other people (like me) simply switch to the flex. hardware when the metal hardware became too uncomfortable and are fine thereon. I have no stretch marks, scarring, etc. (but I'm a minority and our skin is more elastic as well). I swithed to flex hardware when my metal ring became too itchy, then took it out when I went into labor. I didn't put it back in right after delivery but waited until a few months later and had no problem (but I had had the piercing for over 8 years when I took it out). But that was MY experience. The elasticity of your skin, your daily activity level, and the size and shape of your pregnancy are individual "X" factors that will determine your experience of course.
Basically, you may or may not get stretch marks, it may or may not be uncomfortable w/out flex hardware, and your piercing may or may not close up if you don't put it back in after delivery. In general though, the barbell flex hardware worked great for me b/c it is not prone to snagging, you can cut it to whatever length you want (cut it a bit longer to allow for belly growth if you put it on fairly early), and most reputable piercing shops will sterilize your flex hardware and put it in for you for FREE or for a nominal cost. Taking out all hardware just prior to delivery may be prudent though in case an emergency procedure is needed as esp. the metal hardware can possibly get in the way of medical equipment (so I was told).
Lastly, everyone has their own opinion, but I loved wearing form fitting sweaters and tops even when I got big (felt better than wearing a tent like top that just emphasized my girth!). The ends of my barbells did poke out (my belly button never popped so only the barbells poked out) and I thought it was cute! My mother in law was horrified, but like I said, to each her own. In my opinion, with all the involuntary (but yes, wondrous, but still somewhat overwhelming) physical and emotional changes that came w/my pregnancy, keeping my belly piercing was a comforting way to "hold on" to a piece of myself, my identity apart from the "mommy" identity. That was important to me. Do whatever you want, and know that others like me out there support whatever keeps your baby healthy AND makes you happy and sane as well!
Good luck to you and congrats!