What Causes Low Lying Placenta?

My daughter is 13 weeks preganant and has been diagnoised with a low lying placenta, and a hole in the placenta last week after she sarted bleeding. She was put on bedrest for one week, and the bleeding is now down to spot bleeding. She went for a scan yesterday again, and baby is doing well, and the dr has just informed her that she needs to stop all exercise, and not to carry anything heavy. What else must she lookout for and what is the prognoses of this condition.