There are a lot of misunderstandings and confusion out there when it comes to knowing the terms that apply to pregnancy. Most people, for example, don’t realize that a human pregnancy is not actually 40 weeks in duration, which makes some people think they may be further along than they are. Below are some of the ways that pregnancies are typically broken down into timespans for calculation purposes:
Gestational Age (GA): The duration of pregnancy is 40 weeks from the first day of the last menstrual period (LMP.) But, this is only an assumption as many women are not pregnant on the LMP. A more accurate method of dating a pregnancy is fetal age pregnancy dating.
Fetal Age (FA): When a child is in the womb, it develops rapidly. For example, before a baby is even born, he is already producing breast milk and hair. It’s important that you are aware of the fact that the fetus grows at an extraordinarily rapid rate during this time and therefore, to account for this growth, your doctor must make a more accurate prediction regarding childbirth by considering important details like how much weight and length the baby has gained since being in your womb.
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 day that it’s expected that the baby will be born. Generally, ETA is the same as estimated date of confinement (below) but is often used when a pregnancy progresses from knowing when conception happened.
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 article has given some insight into pregnancy dating terminology. Unfortunately though, understanding of how the female reproductive system works or what is even taking place during menstruation is largely influenced by a lot of unscientific thinking and assumptions. We’re going to have to wait before ideas like the idea that ovulation happens on day 14 out of 28 days and that a healthy pregnancy lasts 40 weeks can be easily corrected. As noted in 1977 by Dr. Bruno Walter, one expert on human reproduction (1):
“From archaic experience, women know that their ‘monthlies’ recur in approximately four-weekly intervals. Philosophers and physicians, 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 a 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.