The date a pregnant woman is due to deliver (estimated date of confinement – EDC) can be calculated in the following way:
Add 7 days to the day the woman’s last period started, and 9 months to the month of her last period. For example, if the last period started on 5 January 2003, she will be due to deliver on 12 October 2003. This is also known as Nägele’s rule, after the German obstetrician who first calculated it.
A pregnancy lasts 40 weeks (280 days) from the beginning of the woman’s last period, but only 38 weeks from conception, because she ovulates two weeks after her period starts. It is not unusual for the pregnancy to be one or two weeks shorter or longer than this.