I had placenta prévia with my previous pregnancy. I did the worst thing possible and googled it so then I was terrified. My placenta finally moved up around week 30. I think the biggest concern is that if the placenta doesn't move up you have to have a c section because the baby won't be able to make his way out of the birth canal if the placenta is blocking it. Also, in some cases bleeding can occur.
If your placenta doesn't move and stays by the cervix then you'll likely have to have a c section. Other than that it usually doesn't cause many complications.