I have Placenta Previa (both at 18 then 22.5 weeks). My Dr doesn't seem super worried. If I start bleeding, I bust it to the ER. I get rechcecked again at 32 weeks, and if it remains, there might be restrictions, depending on my "situation". I have a 50% chance of it moving, it was about 1cm away at 22.5 weeks. If not, then I'll have a c-section at 37 weeks, which wouldn't be the end of the world (hey 3 less weeks of uncomfortable pregnancy).
I stressed my entire first pregnancy and now I know that all the worry in the world and all of the internet research isn't going to help the situation. Don't spend too much time on the "What ifs" wait for your Dr to tell you whether to worry or not.