Why is pregnancy based off of your last period?

I don't get how they base pregnancy off of the last period if you have to miss one in order to know if you are? And if conception happens 14 days after the last period? And if it takes 10 to 14 days to have implantation occur? I am so confused. Someone please tell me how they base this.|||Conception doesn't have anything to do with your period - it is what happens if you have unprotected sex. Ovulation happens approx 14 days after the last period or halfway through your cycle. Ovulation releases the egg for possible conception so they use the last period to gauge approx when you would have been ovulating vs when you had sex to estimate the time of conception and estimated delivery date.