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 LMP.

History

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 LMP by adding 1 year, subtracting three months and adding seven days to that date. The result is approximately 280 days (40 weeks) from the LMP.

Example:
LMP = 8 May 2009

+1 year = 8 May 2010

-3 months = 8 February 2010

+7 days = 15 February 2010

280 days past LMP 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 Tuesday. The calculated date (15 February) is a Friday; adjusting to the closest Tuesday produces 12 February, which is exactly 280 days past 8 May. The calculation method does not always result in a 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 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 9 months to LMP, subtracting 21 days, then adding duration of previous cycles.^[1]

In modern practice, calculators, reference cards, or sliding wheels are used to add 280 days to LMP. Many calculators are available online. [1] [2] [3] [4] [5]

Accuracy

LMP may not be the best date to use as the basis of a due date calculation, but it remains popular because few women know exactly what day they ovulate or conceive a pregnancy, and because no algorithm can predict the exact day that spontaneous labor will occur no matter what considerations are taken into account.

Average gestation of 40 weeks

Naegele's Rule presents 280 days after LMP 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:

A standard deviation diagram. When applied to human gestation length, the curve's center is at 280 days (40 weeks) past LMP. The dark blue area shows births within 13 days of the EDD

• 281 days, with a standard deviation of 13 days, was the result of a population-based study of 427,581 singleton births in Sweden.^[2]

• 281 days for first-time mothers and 280 days for all others were the medians found by a 1995 American study of 1,970 spontaneous births. Standard deviation was between 7–9 days.^[3][4]

• 282 days was recommended for cases where LMP is the only known factor in a study of 17,450 patients combining LMP and ultrasound measurement techniques.^[5]

• For first-time mothers, a median of 288 days (274 days from the date of ovulation), was suggested by a 1990 study of 31 private-care patients. The relatively small sample size may be one reason for the larger than normal deviation from 280 days.^[6]

Given the fact that these gestation lengths are only estimates of an average, it is helpful to consider gestation time as a range of dates rather than a single "due date". The median found by Naegele's Rule is merely a guideline for the day at which half of all births occur earlier, and half of all births occur later. Births rarely occur on a due date, but they are clustered around due dates.^[7]

For instance, a standard deviation of 13 days means that 90% of babies will be born within three weeks of their EDD, and 21% will be born within 3 days of it. Only 4% of births will occur on the EDD, but this is similar to most other days around the EDD, in fact each day within a week of the EDD has a 3-4% probability of being the day that birth will occur. However, any given day two weeks away from the EDD has a less than 2% chance of being the day that birth will occur.

Ultrasound confirmation of gestational age

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.

References

1. ^ Rakesh M. Parikh (2007). "Parikh’s formula to minimize errors in calculating expected date of delivery". Medical Hypotheses 68 (4): 928. http://www.medical-hypotheses.com/issues/contents?issue_key=S0306-9877%2807%29X0362-1. 
2. ^ Bergsjø P, Denman DW 3rd, Hoffman HJ, Meirik O. (1990). "Duration of human singleton pregnancy. A population-based study.". Acta Obstet Gynecol Scand: 197–207. 
3. ^ Gray, H (1962). "Duration of pregnancy.". Stanford Medical Bulletin 20: 24–8. doi:10.1046/j.1469-0705.1995.06050353.x. PMID 13901307. 
4. ^ Dr H. Kieler, O. Axelsson, S. Nilsson, U. Waldenströ (1995). "The length of human pregnancy as calculated by ultrasonographic measurement of the fetal biparietal diameter". Ultrasound in Obstetrics & Gynecology 6 (5): 353–357. doi:10.1046/j.1469-0705.1995.06050353.x. PMID 13901307. 
5. ^ Nguyen T, Larsen T, Engholm G, Møller H (1999). "Evaluation of ultrasound-estimated date of delivery in 17,450 spontaneous singleton births: do we need to modify Naegele's rule?". Ultrasound Obstet Gynecol 14 (1): 23–8. doi:10.1046/j.1469-0705.1999.14010023.x. PMID 10461334. 
6. ^ Mittendorf R, Williams MA, Berkey CS, Cotter PF. (1990). "The length of uncomplicated human gestation.". Obstet Gynecol 75 (5): 929–32. PMID 2342739. http://journals.lww.com/greenjournal/toc/1990/06000. 
7. ^ Evaluation of Gestation at eMedicine