Belly button piercing is rather safe procedure if done in sterile place by professional skillful piercer but it's not recommended to get the navel pierced during pregnancy. At first two weeks it is acceptable but it's better to wait for a year and make belly button piercing in better period when your body is ready.
If you already get your belly pierced you have no reasons to worry. The majority of women successfully wear jewelry for pregnant belly. But there are some cases when piercing flattens and stretches too much and it's better to take the jewelry out. It depends on the form of your navel and the sickness of the skin. But such cases are rather rare.
During pregnancy one should better wear jewelry made of flexible bioplast. There is a wide choice of super long flexible barbells and maternity belly rings made of such materials wearing which any pregnant woman will feel comfortable and safe. Of course you should take care of your belly and in case you had just got your navel pierced and then found out that you'll become a mother you should make a decision to abandon the piercing or not. If the healing process is almost over you can leave it and follow all aftercare recommendations extremely thoroughly. But it's better to choose the abandoning of the piercing for the sake of your baby's and your health.
After the delivery your navel will get its pre-pregnancy shape and you will return the possibility to enjoy your piercing as before. But there are some cases when the skin round the navel looses and piercings sags. Then you'd better consult your doctor or the professional piercer.
In all cases you don't have to worry. If you do your best to take a proper care of your pierced belly everything will be all right. Enjoy your pregnancy and your extremely beautiful belly.