Infection is a dangerous thing for both the pregnant woman and for her developing baby. Any activity that puts your body at an increased risk for infection is generally not considered a good idea during pregnancy. Getting piercings anywhere on your body, including your belly button, ears, genitals, or nipples, puts you at an increased risk for infection. Pregnant women are considered particularly vulnerable to infection, because their immune system is generally weaker and trying to work harder than usual while you are pregnant.
However, if you have a belly piercing, navel ring, or some other piercing when you become pregnant, there is not necessarily a general health reason for removing your piercing while you are pregnant. Unless you have a fresh piercing that may be vulnerable to infection, piercing should not cause you any trouble during your pregnancy. You will want to watch out for the risk that these recent piercings can present in terms of putting you at a higher risk for infection.
For women who have their belly button pierced, they may find that it becomes more difficult to wear their navel ring during their pregnancy. This is especially the case during the later stages of pregnancy, once their belly button pops out. If you have had your belly piercing for three years or more, you can feel free to just take it out. It is much less likely at this point that the hole will close. If your belly piercing is relatively new, you might need to keep something in there. For some women who have a navel ring, a more comfortable piece of jewelry will do the trick. In fact, several jewelry manufacturers are now making navel rings that are specifically designed for the pregnant, belly pierced woman. If another piece of jewelry does not do the trick, you can replace your belly ring with a small clean piece of fishing line. The fishing line will flex as your belly grows.
As always, it is a good idea to talk to your health care provider about specific pre-existing piercings, including a navel ring, while you are pregnant. There may be unique health concerns that your health care provider knows of that should be adequately addressed.