Conception and calculations??

This may just be an utterly dumb question, or many women may be confused by this.....or it could most likely just be me confused... but. When dating back to how many weeks along you are, what do you go off of? Is it the date that you actually had sex, and he came inside, that you go off of, or is it based off the first missed period? Or is it when the egg and sperm actually "connect" that you start counting off? If someone could set me straight, that'd be completely awesome of yall.