Placenta previa?

Im 21 weeks and I've been having very light spotting last week (had it a little around 12 weeks too). The ultrasound showed I have a posterior placenta but didn't note if it was low lying. My Dr is concerned that it may be close to my cervix causing the bleeding. She said it might be placenta previa. This is my second child and I didn't haven't this with my first. Has anybody had placenta previa and if so, what was the treatment?