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.