Naegele's rule

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 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 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 fertilization.

Example:
LMP = 8 May 2007

+1 year = 8 May 2008

-3 months = 8 February 2008

+7 days = 15 February 2008

or +9 months and +7 days only (this way the year will change automatically).

(280 days after 8 May 2007 would be 12 February 2008.)

The rule can be made fully precise 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 2007 is a Tuesday. The calculated date (15 February) is a Friday, adjusting to the closest Tuesday produces 12 February, which is the correct date.

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 Naegele'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. (However the adjustment described above can produce precise dates.)

Certain criteria should be followed for applying Naegele's rule:

1. Previous 12 cycles must be regular and 28–30 days cycle;
2. The previous 12 regular cycles must not be by the use of oral contraceptive pills;
3. The last menstrual period must be normal, ie. the duration of menstrual bleeding 3–5 days and average number of pad changed per day is 3;

Average gestation

The rule assumes 40 weeks, or 280 days from LMP (266 days from the date of ovulation). Studies of uncomplicated spontaneous-labor pregnancies have shown that this assumption leads to due dates that are premature, relative to the median. One recent study suggests that the EDD would be better estimated using 288 days from the LMP date (40 weeks and 8 days, or 274 days from the date of ovulation) for first pregnancies and 283 days from the LMP date (40 weeks and 3 days, or 269 days from the date of ovulation) for subsequent pregnancies.^[1]. Another study suggests modifying Naegele's rule by using 282 days from the LMP date (40 weeks and 2 days, or 268 days from ovulation),^[2] although this is not 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 1970s 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, Canada, 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.

In Clinical Practice

This section may require copy-editing.

Only 3-5 percent of uncomplicated singleton pregnancies deliver spontaneously on the exact day as mentioned by this more than 150 year old "Rule" which finds itself obsolete in the form it was stated. It is quite unfortunate and tragic, though Naegele and Gauss were contemporaries the concept of physiological variation in natural duration of Pregnancy is not well perceived and applied in clinical practice. The standard deviations around a mean value of gestational age has to be considered for a particular population cohort. Even the subsequent pregnancies in same woman almost never obey this rule. Neither the ultrasound machines determine the exact day of end of gestation though it can be expressed in terms of mean +/- standard deviation of few weeks apart. The gestational age is unique to each feto-maternal unit. There are ethnic variations in mean gestational age too. Ideally there is a 6 weeks confidence interval anytime from 36 to 42 weeks during which spontaneous normal labor is taking place with complete functional maturity. This is the concept of individual term (Let man be born at his own due time-Rudolf Klimek).^[3],^[4]

Functional Maturity of the Fetus is an independent time spatial event that follows auxonomic laws and quantam mechanics.[1] Six-week period of birth occurrence in a human being is too large for obstetrical management to be determined by statistical methods. Fetal maturity quanta is expressed in Vertical time unlike the USG based measurements which are widely based and biased on horizontal time. During clinical contemporary practice, even the most sophisticated ultrasound imaging equipment fails to provide more accurate information concerning individual birth dates, than does Naegeles Rule (95% confidence interval ± 3 weeks).[2]

The end of Gestation is important than beginning. The risks of iatrogenic prematurity and lung disease for those infants delivered by elective CS before labor are 30 percent versus those born by cesarean after labor begins (11 percent). At least 3% of preterm births occur beyond 37 gestational calendar week. This percentage is doubled owing to induction of labor only because the observed pregnancy has reached its 287th or 294th day, which unfortunately is the case in many countries where postdatism creates iatrogenic “prematurity”. A delivery planned at an inappropriate time can increase the average total blood loss. There are gestational age and birth weight independent scoring systems and computer aided methods ^[5] to know the functional or biological gestational age of the fetus.Klimek and Ballard scoring systems are the commonly used for the postnatal assessment of Gestational age.[3],[4],[5],[6]

References

• Evaluation of Gestation at eMedicine

Footnotes