Naegele's rule

Overview
Naegele's Rule is a standard way of calculating the due date for a pregnancy. It is named after Franz Karl Naegele (1778-1851), the German obstetrician who devised the rule.

History
Franz Karl Naegele was born July 12, 1778, in Duesseldorf, Germany. In 1806 Naegele became ordinary professor and director of the lying-in hospital in Heidelberg. His "Lehrbuch der Geburtshuelfe," published in 1830 for midwives, enjoyed a successful 14 editions.

The rule estimates the Expected Date of Delivery (EDD) from the first day of the woman's Last Menstrual Period (LMP) by adding a year, subtracting three months and adding seven days to that date. This approximates to the average normal human pregnancy which lasts 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from the date of fertilisation. Example, if LMP was 8 May 2007, then +1 year = 8 May 2008, -3 months = 8 February 2008, +7 days = 15 February 2008; whereas precisely 280 days would be 12 february 2008.

Assumptions
This method is a fairly good estimation of the due date, but makes a number of assumptions.


 * An average menstrual cycle: The rule is based on ovulation and fertilization happening on cycle day 14 with a cycle length of 28 days. It is less accurate when ovulation happens earlier or later in the cycle, and for women with irregular menstrual periods. Sometimes women may have a light bleed of a threatened miscarriage in early pregnancy, which adds to this confusion if it is mistaken for a period.


 * Average lengths of calendar months: The rule assumes an average length for all months in the Gregorian calendar year of just over 30 days (365 / 12 = 30.416). The rule, on average, therefore equates to 280.75 days. However the unequal length of the Gregorian calendar's months results in Naegels's rule overestimating the EDD by 3 days for all LMPs in May, and depending on the effect of leap years gives precise dates on only either 2, 5 or 7 months of the year.


 * Average gestation: The rule assumes 40 weeks, or 280 days from LMP. Recent studies have suggested that the EDD would be better estimated using 282 days from the LMP date (40 weeks and 2 days), this is not though current accepted clinical practice.

Other estimation techniques
The effects of varying lengths of the calendar months may be overcome by use of hand operated pregnancy wheel devices, or the use of electronic computer pregnancy calculator programs.

Ultrasound confirmation of gestational age
Since the estimation is based on the LMP date, Naegele's rule can easily be a week or more off, it should be used to calculate a range of due dates and confirmed with ultrasound in the first trimester. Since the 1970's ultrasound scans have allowed measurement of the size of developing embryos directly and so allow for an estimation of gestation age. Ultrasound dating is most accurate if undertaken in the first trimester (first 12 weeks of pregnancy) with a 95% error margin of 6 days. Scans performed in the second trimester have an error margin of 8 days and those in the third trimester a margin of 2 weeks.

Most obstetric departments in Australia, United Kingdom and United States use a combination of LMP and ultrasound based estimates for the EDD using either 10-day or 7-day rules, so that if LMP dates and ultrasonographic dates are in agreement within 7 (or 10) days, then the LMP dates are accepted.