The expected date of delivery (EDD) is calculated by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP). The result is approximately 280 days (40 weeks) from the LMP.
LMP = 8 May 2009
+1 year = 8 May 2010
=3 months = 8 February 2010
+7 days = 15 February 2010
The calculation method above does not always result in a 280 days because not all calendar months are the same length, not does it account for leap years.
In modern practice, calculators, reference cards, or sliding wheel calculators are used to add 280 days to LMP.
LMP may not be the best date to use as the basis of a due date calculation, but it remains popular because few women know exactly what day they ovulate or conceive a pregnancy, and because no algorithm can predict the exact day that spontaneous labour will occur no matter what considerations are taken into account.