All About Due Dates

What Is Naegele's Rule?

A German physician named Franz Carl Naegele [1777-1851] was the first person to scientifically study and write about estimating due dates. He developed a rule, now called Naegele's Rule, for estimating a pregnant woman's due date.  Naegele's rule states that the due date can be found by taking the first day of a woman's last menstrual period, subtracting three months, adding one week and adjusting the year if necessary.  For example, if a woman's last menstrual period was April 14th, 2002 then her due date by Naegele's rule would be January 21st, 2003, i.e.:

            April           14,       2002
              - 3 months   + 7 days    + 1
       -----------------------------------
       =  January           21,       2003

Subtracting 3 months from April gives you January in the above example.  Adding 7 days to the 14th gives you the 21st.  Since you can't go backwards in time to deliver before your last menstrual period, you have to adjust the year by adding one to 2002 to get 2003. 

Modern studies have shown that Naegele's Rule is remarkably accurate.  Yet, few care providers actually use Naegele's Rule today because it's not accurate enough by today's standards.  When thousands of pregnancies are studied, the average length of pregnancy from a reliable first day of last menstrual period until delivery is 280 days.  The modern way to estimate a due date is to 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.  Note that, for a last menstrual period of April 14th, 2002, there is a two day difference between the due date calculated by Naegele's Rule (January 21st, 2003, see above) versus the due date calculated by adding 280 days to the last menstrual period (January 19th, 2003).

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 "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.