Yes, you'll be fine wearing one. Buying plastic belly button rings may be better to wear than metal ones because as your belly expands the plastic can bend with it. My mom had hers in with my sister and so did my friend when she was preggers. Nothing bad happens to it(:
It's not healthy to get one while you are but it is alright to wear it while youre pregnant. But if you have doubts then just remember that if you take it off then the hole can close before you give birth.
You will probably get to a point in the last month where it becomes uncomfortable, easy to snag, pull, stretches. Get some fishing line, tie a ring, it will keep the hole open if that's what you are concerned with. Congrats!