All About Due Dates

How do you calculate due dates?

When thousands of pregnancies are studied, where the first day of the last menstrual period is known for certain, the average length of pregnancy from the first day of the last menstrual period until delivery is 280 days.  To calculate a due date, you simply add 280 days to a reliable first day of last menstrual period.  If you add 280 days to a last menstrual period of April 14th, 2002, you get a due date of January 19th, 2003.  This method results in a slightly different due date than you would get using Naegele's Rule, but virtually all modern care providers use this method.

Any due date calculation based on a last menstrual period will only be as accurate as the date of the last menstrual period.  Sometimes women are unsure about what is ment by "last menstrual period."  A common mistake is to use the LAST DAY of the last menstrual period.  Due dates are calculated from the FIRST DAY of the last menstrual period.  Another common mistake is to use the date that a woman FIRST MISSED her period, that is, the date that a period was expected, but never started.  Due dates are always calculated from the first day of the last menstrual period, not from the date of the first missed period.

Other factors might make a due date calculation less accurate.  Sometimes women are unsure about the exact date of their last menstrual period.  The best-case scenario is that you are one of those compulsive women who marks your periods on your calendar and can simply consult your calendar.  The second best scenario is that there was some circumstance surrounding your last menstrual period that allows you to nail the date, for example, your last menstrual period started on the first day of a romantic Caribbean cruise. 

It's important to realize that a due date is an estimate.  Even though the average length of a pregnancy is 280 days, few pregnancies are exactly 280 days in length.  For any average, there is a range on either side of that average that is still considered normal.  For example, the average height of men in the United States is 5 feet, 10 inches.  But, most perfectly normal American men are taller or shorter than 5 feet, 10 inches.  By the same token, even though the average length of a pregnancy is 280 days, most normal pregnancies are longer or shorter than 280 days.  However, about 80% of all pregnant women with a reliable last menstrual period will deliver within 14 days either side of 280 days from the first day of their last menstrual period.

It's not easy to add 280 days to a last menstrual period on paper, but there are devices that can be used for this calculation.  You may have seen your care provider using a round, plastic or metal device called a "gestation wheel," or more often a "wheel" to calculate your due date.  Many patients ask where they can get one of these devices.  An electronic Due Date Calculator is available on this website.