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
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.
Ultrasound is often used to estimate a due date based on the size of the fetus.
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
Sarah Marshall, MD - Family Medicine & Kathleen Romito, MD - Family Medicine & Adam Husney, MD - Family Medicine & Kirtly Jones, MD - Obstetrics and Gynecology