My obgyn said i'm 10 weeks but if i count to the time i had intercourse it adds up to eight weeks. where did two extra weeks come from?

Last menses: Pregnancies are dated based on the first day of the last menstrual period. It makes sense if you realize that this system was developed before we understood the ovulation process, and the only criterion that was known was the last period. This system means you are 2 weeks pregnant at the time of conception and 4 weeks when the pregnancy test is positive.