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More information about the Mittendorf-Williams rule
This study done in 1990 showed that pregnancy lasted an average of 288 days past last menstrual period (LMP) for Caucasian first-time moms. For Caucasian women who were not first-time moms, their date of delivery averaged 283 days past LMP (3 days after Naegele’s rule predicted). This finding is known as the Mittendorf-Williams rule.
Our advanced due date calculator above is based on the Mittendorf-Williams rule.
Whereas our basic due date calculator uses “Naegele’s rule,” which predicts childbirth to occur 280 days after the first day of the last menstrual period. While Naegele’s rule is still the most widely used for calculating pregnancy due dates, the Mittendorf-Williams rule is proving to be more accurate.
Read more about how pregnancy due dates are calculated on our basic due date calculator page.