R.W.
If it is clearly old blood I wouldn't freak out too badly. I have had some spotting early is pregnancy and while it is scary to think of the what ifs. MOST spotting resolves itself.
If you are crampy, or you start seeing pink or red discharge then I would call your OB/Midwife. For advice. Unfortunatly at this point in pregnancy there isn't anything they can do to help.
Lay down, put your feet up, rest eat and drink normally. Take it easy to avoid anymore bleeding. And don't worry about where the placenta is at this point. You still have PLENTY of time for that Placenta to move up and out of the way.