Hi!
If you were my patient, I would try to explain it to you this way:
MOST women don't know their exact conception date, or there could be more than one option SO for that reason, we calculate the pregnancy dates based upon the l first day of the last menstrual period. SO since your last menstrual period was presumably about 2 weeks prior to your ovulation date which is probably your also your conception date--that is where the extra two weeks comes from.
Hope this helps!
Dr B
Yes that's exactly how it works :) Lol confusing isn't it ? I didn't understand it either at first