Well my due dates go like this. First baby was due 16 March, at first scan they changed her to 11 March, second scan back to 16 March. She arrived 23 Feb. I wasn't sure of my dates with her, so I can't tell you when I thought she was due.
Second baby was due 23 March, her first scan was pretty much spot on with due date. second scan they moved due date to 20 March. She arrived 11 March. I was sure with my dates with this baby and so I guess she just arrived early.
My third I know is due on the 2 March 2005, and so far I haven't had a proper scan done, but the Dr has done two scans and he has come up with 2 March both times.
I guess the hospital and ultrasound places tend to go on the lengths etc of the baby, but that doesn't mean anything because you could be tall people and your baby be longer than my children, but your baby could still be born on its original due due.
If you are really sure of your dates, just stick to that due date. They also allow that two week period because most people don't know exactly when they conceived their baby.
Good luck with your pregnancy/baby.
QLD DD 2/02 DD 3/03 DD 2/05 Jack 23/3/07