Probable date of ovulation December 13 and possible dates of conception are December 9-17
Your doctor didn't tell you that? Just use a pregnancy calendar online
At what point in pregnancy did you get your due date, and how was it determined? If it was by an early ultrasound (within your first trimester), the dates given by Dominique will be reliable. If it was just computed on a calendar from the first day of your last period, they might not be as reliable.