Due date confusion

The day of my last period was Nov 16, 2018. Which according to that my due date is Aug 23, 2019. The day I conceived was Dec 4, 2018 which says my due date would be Aug 27, 2019. If it’s 40 weeks from your last period wouldn’t it be 38 weeks from the conception date?

I’m so confused. My doctor is no help. Lol