Basically due dates are worked out on the assumption everyone has a perfect 28 day cycle, and ovulates on day 14 (whereas for this pregnancy I ovulated on day 54!), and so the due date is 40 weeks after the first day of your last period.
So if you know you conceived on 1st November, technically you would be classed as 2 weeks pregnant by then (using the perfect cycle assumptions). I always know date of conception as I have to track ovulation, so when midwives ask for the date of my last period I just give them the date two weeks before that, and keeps everyone right. For this baby I tell the, my LMP was 20th July when it was actually waaay back in June!
And congratulations!