A typical pregnancy lasts, on average, 280 days, or 40
weeks - starting with the first day of your last normal
menstrual period (LMP) as day 1. An estimated due date
(EDD) can be calculated by following steps 1 through 3:
First, determine the first day of your last menstrual
period. If you do not know the exact date, calculate
as close as possible.
count back three calendar months from that date.
add one year and seven days to that date.