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This advanced due date calculator utilizes the Mittendorf-Williams rule, which factors in more info about you to provide more accurate results.
What is the Mittendorf-Williams Rule?
Named for Robert Mittendorf, assistant professor and director of health studies in obstetrics & gynecology, and Michelle Williams, an epidemiologist at the University of Washington. the Mittendorf-Williams Rule uses 16 significant factors—maternal age, pre-pregnancy weight, race, college education, alcohol and coffee use, hypertension, and other medical conditions—that provide a more accurate way of dating pregnancy and identifying whether a woman is at risk for preterm delivery. (source)
The Mittendorf-Williams rule is based on two studies:
- This study done in 1990 that showed 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).
- The second study done in 1993 suggests that there are several factors, including number of previous births, age, and race that influence the length of pregnancy.
Until now, there has been no successful way to determine numerically the risk for preterm delivery. By identifying these women, we may be able to lower the infant mortality rate by intensifying a woman’s prenatal care, and medically manage patients differently based on this new information. — Robert Mittendorf
Though few women give birth on their actual due date, a prospective study comparing the Mittendorf-Williams Rule to Naegele’s rule found the Mittendorf-Williams Rule to be twice as accurate in predicting gestation. (source)
Advanced Due Date Calculator vs. Basic Due Date Calculator
- If you know how long your luteal phase is, use our advanced due date calculator above, which is based on the Mittendorf-Williams rule, to get a more accurate due date.
- If you don’t have this information, use our more basic due date calculator, which follows “Naegele’s rule” and predicts childbirth to occur 280 days after the first day of the last menstrual period.
In other words, though Naegele’s rule is still the most widely used for calculating pregnancy due dates, the Mittendorf-Williams Rule and our advanced due date calculator will provide the most accurate answer to the burning question of, “how many weeks pregnant am I?”
Still, it’s important to remember, no matter what tool you use, a due date is just an estimate. Read more about how pregnancy due dates are calculated (and why they aren’t set in stone) on our basic due date calculator page.
When Did I Conceive?
If you already know your due date and wish to know when you likely conceived, try our reverse due date calculator. As long as you know your due date (or birthday), the tool will calculate your estimated conception date and intercourse dates.