Expected delivery date
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.
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.
Calculation
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
LMP = 8 May 2009
+1 year = 8 May
2010
−3 months = 8
February 2010
+7 days = 15 February
2010
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.
Accuracy
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 because few women know exactly on which day they ovulated or conceived 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
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:
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 after
LMP with a standard deviation of 13 days, was the result of a
population-based study of 427,581 singleton births in Sweden.
·
281 days after
LMP 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 7–9 days.
·
282 days after
LMP was recommended for cases where LMP is the only known factor, in a study of
17,450 patients combining LMP and ultrasound measurement techniques.
·
A median of 288 days (274 days from the date of ovulation)
for first-time mothers and 283
days(269 days from the date of ovulation) for mothers with at least one
previous pregnancy was found by a 1990 study of 114 white, private-care
patients with uncomplicated pregnancies and spontaneous labor. The authors
suggest that excluding pregnancies involving complications (that often lead to
pre-term deliveries) accounts for the longer periods.
Given that these
gestation lengths are only estimates of an average, it is helpful to consider
gestation time as a range of dates rather than as 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 clusteredaround due
dates.
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 three days of it. Only 4%
of births will occur on the EDD, but this is
similar to most other days around the EDD; each day within a week of the EDD
has a 3 to 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
six days. Scans performed in the second trimester have an error margin of 8
days and those in the third trimester a margin of two weeks.
Most obstetric departments
in Australia, Canada,
the 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.
মন্তব্যসমূহ
একটি মন্তব্য পোস্ট করুন