I'm rather at a loss what to write, as I've already said in my comments to Ana's and Toni's blogs that I don't really know what is acceptable as a prologue and what isn't.
However, I did 'google' the subject, and came up with this article which I thought was quite interesting, even if it didn't completely clarify my confusion about prologues:
A couple of sentences in this caught my eye: "Perhaps they [key facts or other information] occur in another time or place, and have too much weight to mention by-the-by. Or they might choke the narrative to death with background details."
I have this problem in my first novel (yes, you know the one, M & B have had it for 10 months!). At it stands at the moment, I've included 'chunks' of backstory because it's necessaary for the reader to understand what happened 5 years earlier. But I've always had the feeling that it 'chokes the narrative' - so now I'm wondering if I should put the backstory as a prologue, or simply call it Chapter One.