K.E.
With my first baby, I was dilated to almost a 3, and baby was very low about 3 weeks before I was due. With my second, I was only dilated to 1 starting at 36 weeks, and she stayed high right up until I was in late labor. I actually felt her drop, and it made me throw up. It was a very strange sensation :D
With my third and fourth babies, I think they stayed pretty high, right up until I was going into labor. I don't recall how dilated I was. With my third baby though, I was due August 9, 2009, and he was born on July 24. My labor started at home, I was not induced, so it was just his time to be born. Due dates are not always accurate, and baby will come when your body is done making him/her. I wouldn't worry so much about your conception date. If you had any first trimester ultrasounds, I'm sure that's how they are calculating your due date, and they are usually pretty good at dating a pregnancy.
I know it's frustrating when doctors blow off your questions, and they give the generic "that's normal" answer to everything. You know your body best, and if you think you're going to deliver earlier than your doctor believes, then definitely prepare for that, because you're probably right. Good luck. I know it's hard not to get antsy toward the end.
K.