@mommytobeanny, my due date has been the same since day one, I even calculated my own due date before my first prenatal appointment. They can really determine exactly how far you are and when you're due when you go to an ultrasound.
with this baby I went to a free clinic for an ultrasound and they tried to give me a different due date than what I already knew was right. But it was only different by 2 days, when I went to my regular doctor they changed it back.