Hi Jess,
In my recent pregnancy, there were a number of key dr's appts where the EDD was discussed and agreed upon, mostly in the early 2nd trimester, and I suggest when this topic comes up again, tell the dr of your previous experience, and also your certainty with your own dates. *Everyone* knows that the ultrasounds are only an indication and should not be leaned upon too heavily unless there is some existing doubt as to dates (ie if you have a regular cycle and are aware of when you ovulated that is different from someone with irregular bleeding and no clear LMP). If you're assertive at the beginning of your pregnancy you shouldn't have any problems near the end.