Naegele's rule

Naegele's rule is a standard way of calculating the due date for a pregnancy. The rule estimates the expected date of delivery (EDD) by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period (LMP). The result is approximately 280 days (40 weeks) from the start of the last menstrual period. Another method is by adding 9 months and 7 days to the first day of the last menstrual period.

Naegele's rule is named after Franz Karl Naegele (1778–1851), the German obstetrician who devised the rule. Naegele was born July 12, 1778, in Düsseldorf, Germany. In 1806, Naegele became ordinary professor and director of the lying-in hospital in Heidelberg. His Lehrbuch der Geburtshilfe, published in 1830 for midwives, enjoyed a successful 14 editions.

The rule estimates the expected date of delivery (EDD) (also called EDC, for estimated date of confinement) from the first day of the woman's last menstrual period (LMP) by adding one year, subtracting three months, and adding seven days to that date. The result is approximately 280 days (40 weeks) from the start of the last menstrual period.

Example:
LMP = 8 May 2009

280 days past the start of the last menstrual period is found by checking the day of the week of the LMP and adjusting the calculated date to land on the same day of the week. Using the example above, 8 May 2009 is a Friday. The calculated date (15 February) is a Monday; adjusting to the closest Friday produces 12 February, which is exactly 280 days past 8 May. The calculation method does not always result in 280 days because not all calendar months are the same length; it does not account for leap years.

Parikh's formula is a calculation method that considers cycle duration. Naegele's rule assumes an average cycle length of 28 days, which is not true for everyone. EDD is calculated using Parikh's formula by adding nine months to the start of the last menstrual period, subtracting 21 days, then adding duration of previous cycles.

In modern practice, calculators, reference cards, or sliding wheel calculators are used to add 280 days to the LMP.

The date on which the last menstrual period began may not be the best date to use as the basis of a due date calculation, but it remains popular. The continued use of Naegele's rule, rather than more precise estimates such as the linear model derived in 1993 by Mittendorf, et al., attracted criticism in Super Crunchers.

Naegele's rule presents 280 days after start of the last menstrual period as an estimate for the average onset of spontaneous labor. A number of studies have been published in recent years to support continued use of this number:

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