A due date is an estimate of when a pregnant woman will have her baby. It is calculated by taking the first day of her last menstrual period (LMP), adding 7 days, and then counting backward 3 months; or by adding 40 weeks from the first day of the last menstrual period (for women with regular 28-day cycles).
For example, using the first method, if a pregnant woman's last period started on March 20, add 7 days to get March 27 and then subtract 3 months to get a due date of December 27. Using the second method, 40 weeks counted from March 20 is December 25.
The due date is only an estimate of when a woman will deliver. Most women do not give birth on their due date, but they do within 2 weeks before or after.
WebMD Medical Reference from Healthwise