I think you probably need to talk to your ob about your particular circumstances as I know of different people who have got different advice for different severities i.e. the placenta can be near the cervix, over it a bit, over it half way, over it over half way, completely covering it. On one hand I have heard of people being put on complete bed rest, on the other hand I know some one who didn't have it diagnosed and had a normal pregnancy and went into labour 'normally' (but had to have a c-section obviously).

Good luck.