To account for the two weeks prior to conception, doctors take the first day of your last menstrual period (LMP) and add 280 days (or forty weeks) to determine the due date. From there, you can also determine how far along you are by counting how many weeks it has been since the first day of your last period.

How do doctors calculate pregnancy weeks?

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.