The most common method of calculating a due date takes the first day of a woman's last menstrual period, adds seven days, and counts backward three months, according to WebMD. A woman can also estimate her due date by adding 40 weeks to the first day of her last period.
As a general rule, the due date is 280 days, or approximately 10 months, after the first day of a woman's last menstrual period, according to WebMD. However, due dates vary based on the normal length of the woman's menstrual cycle, irregular periods and other factors. Although the due date is only an estimate, most women deliver within 14 days of their due dates.