I'm pretty sure I got pregnant December 26th but my doctor but my due date at Sept third. Is September third a possible due date if the conception date was December 26th?

You go back 3 months and add a week, so December 26th - 3 months is September 26, plus 7 days is October 2nd. But the doctors always go by the date you give them of the first day of your last period. You would be due September 3rd if the first day of your last period was November 26th.. So, wow, maybe he heard you wrong?