Have you thought maybe the doctor got your due date wrong or you just aren't sure. There is one fun way to double check your due dates.
Watch for the first day that you feel baby kick. Now, add 5 months to that day and consider that as your due date.
It is very hard to know exactly when you conceive and the doctors only guess at the date. This is another way to check your dates.
Resource: Husband-Coached Childbirth (Fifth Edition): The Bradley Method of Natural Childbirth pg. 170.