Pregnancy is counted from the first day of the womans last period, not the date of conception which generally occurs two weeks later.
Why is your pregnancy dated from last period?
If you were having regular periods before pregnancy, your doctor will calculate your due date based off of your last menstrual period. This goes back to the fact that in order to get pregnant, your body ovulated—or released an egg—roughly in the middle of your cycle and it was fertilized by sperm.
How many weeks pregnant are you from last period?
Last menstrual period (LMP): Pregnancy normally lasts about 40 weeks from the first day of your last period. Accordingly, the number of weeks that have passed since indicate what week of pregnancy youre in. To work out your likely due date, count 280 days (40 weeks) from the first day of your last period.