To have a natural third stage what you may need to stay away from is managed labour (inductions) your more likely to have a PPH after an induction. The retained placenta no one knows why it happens to some and not others but pulling on the cord before the placenta is ready to detatch (aggressive management of third stage) may be one cause. Your Ob may see this history as even more reason to manage even more aggressively next time ie a syntocinon drip after the birth rather than just an injection. What have you been reading to get more educated and informed about natural third stage ?