Due date messed up!

9•20•16💙 • Mommy of 3😊 Expecting our 4th 💙
So my lmp was 12/12/15 & according to that my due date is supposed to be 9/17. Dr gave me a due date of 9/20... even if they were going off U/S i still think they may be wrong bcs baby is always growing 3 days ahead of how far they say I am. Im going to say something to my Dr tomorrow... I just thought about that