Pregnancy Dating: Making Sense of the Medical
Terminology
By David
Picella
Despite all the breakthroughs of science and modern medicine the ability
to pinpoint the beginning and end of a pregnancy is still not well
understood. Most people, for example, are shocked to find out that a human
pregnancy is not actually 40 weeks in duration. Here are some common terms
and abbreviations that you may run into during pregnancy along with their
definitions.
Gestation: From Latin, this refers to the period of development in the
uterus from conception until birth.
Gestational Age (GA): The duration of pregnancy starting from the first
day of the last menstrual period (LMP). GA pregnancy dating is based on the
assumption that the average pregnancy is 40 weeks from the first day of the
last menstrual period. GA pregnancy dating is widely used, but is frequently
inaccurate for the simple reason that a woman is not pregnant on the LMP.
Fetal age pregnancy dating is much more accurate.
Fetal Age (FA): The fetal age is the duration of a pregnancy starting
from the time of conception. It is much more accurate to calculate the date
of delivery if the estimated time of conception (ETC) is known. FA pregnancy
dating assumes that a pregnancy will be approximately 38 weeks from the time
of conception, until the time of birth.
Last Menstrual Period (LMP): The first day of the last menstrual period.
Estimated Time of Conception (ETC): The date by which conception is
thought to have occurred.
Estimated Time of Arrival (ETA): The date on which it is expected that
the baby is due to be born. The term ETA is actually identical to the
"estimated date of confinement" (below), but is typically used
when pregnancy dating proceeds from a fairly well known estimation of the
time of conception (ETC).
Formula: ETA = ETC – 7 days + 9 months
Estimated Date of Confinement (EDC): The EDC is the date on which it is
expected that the baby is due to be born. The term EDC is actually identical
to the "estimated time of conception" (ETC). The EDC is the most
common term used to indicate the "due date" for a pregnancy. It is
typically used when the date of the last menstrual period (LMP) is known,
but the estimated time of conception (ETC) is not known.
Formula: EDC = LMP + 7 days + 9 months (Naegele's Rule)
Hopefully, this short article has clarified the correct use of
terminology related to pregnancy dating. Unfortunately, our understanding of
the the events of the menstrual cycle and pregnancy will probably continue
to be influenced by some very unscientific thinking. Addressing the
mythology of the 28 day menstrual cycle and the 40 week pregnancy, Dr. Bruno
Walter, back in 1977 most excellently noted that:
"From archaic experience, women know that their 'monthlies' recur in
approximately four-weekly intervals. Philosophers and physician, however,
translated 'four-weeks' into 28 days. By this legitimate trick, woman's
reproductive physiology got assorted with the cabala of numbers, 4 times 7
for the menstrual cycle and 10 times 28 for the duration of the pregnancy.
To the physician, the 28-day menstrual cycle, as well as the 280 days'
duration of pregnancy became the static, orthodox diagnostic criteria of
woman's health and physical normalcy. For the philosopher, the 28 days was
the bridge to the mythology of the moon. The comparison of the observed
occurrence of high tides and the resolution of woman's plethora in
menstruation with the phases of the moon, produced the first rational theory
of the mystic process of periodic vaginal bleeding in women" (Walter,
1977, p. 1).
...and so it has been ever since.
References
Walter, B. (1977). Theme and variations. In R. F. Vollman., The menstrual
cycle. Philadelphia. W. B. Saunders.
Vollman, R. F. (1977). The menstrual cycle. Philadelphia. W. B. Saunders.
Copyright 2006 David Picella
David Picella is a PhD Student at the Unersity of Milwaukee
Wisconsin and a Family Nurse Practitioner who writes about
women's healthcare and natural family planning. You can read
more articles by David and other medical experts in these areas
at http://picella.com
The Compleat Mother Magazine
