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So the normal gestational period is defined to be "from the first day of the woman's last menstrual cycle to the current date.".

Why is that? Wouldn't the more natural definition be "from two weeks after the first day of the woman's last menstrual cycle to the current date"?

I realize that the first day of the last menstruation is a definite date that can usually be measured, but between that date and the date of conception, normally 14 days, the woman is actually not pregnant. Why are two weeks of non-pregnancy included in the definition of the period of gestation?

This would change the meaning of the normal length of pregnancy from 282 days to 268 days.

robert bristow-johnson
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    I don't have a reference, but I assume its because this dating system pre-dated our understanding of the physiology of conception. Why change a system that worked for hundreds of years if it "ain't broke"? Note also that researchers will sometimes use "postconception age" or "embryonic age" to denote the amount of time that has passed since conception. – Ian Campbell Mar 14 '23 at 19:54
  • And... why does this matter? If it's conventional to define the duration of pregnancy as 40 weeks, what will changing it to 38 weeks do? Medicine is steeped in tradition, and change is slow, though it does eventually come. – anongoodnurse Mar 14 '23 at 19:54
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    @anongoodnurse I am reading a little about this controversy regarding abortion, both medically indicated and elective. In the controversy there are different issues that come up at different moments of the gestational timeline. I am just now discovering a Wiki page that repeats what Ian said, – robert bristow-johnson Mar 14 '23 at 22:56
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    If you're looking at this from the perspective of abortion laws, then your question does involve an important distinction. The law generally looks at weeks of gestation as defined by medicine, and some recent state legislation in the US has banned abortion after just a few weeks of pregnancy. If two of those few weeks aren't actual pregnancy, then such laws may be even more draconian. – Carey Gregory Mar 15 '23 at 04:22
  • Exactly, @CareyGregory . Does an 8-week old pregnancy actually mean that the woman was pregnant for 8 weeks? Or does it mean that the woman was pregnant 6 weeks? Because the embryo at the end of the 8th week in the gestational timeline is actually 6 weeks old. That embryo only existed for 6 weeks. – robert bristow-johnson Mar 15 '23 at 19:09
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    @robertbristow-johnson Furthermore, implantation doesn't occur until ~a week later. – Bryan Krause Mar 16 '23 at 14:13
  • "8 weeks pregnant" is 8 weeks from the last menstrual period. If discussing medically, we use gestational age, which would be stated as "6 weeks gestational age" or gestational age of 6 weeks". @IanCampbell - I don't know about the terminology of some areas of research (e.g. stem cell research), but I've never come across those terms. – anongoodnurse Mar 16 '23 at 15:46
  • Having read your rationale, I do think it would be beneficial to change the terminology. But doctors are, well, trained to think in terms weeks when considering fetal or maternal health. We automatically think in terms of "14 weeks" (time period preceding which most spontaneous losses occur), 28 weeks (viability), 38 weeks (full term), >40 weeks (late term), 42 weeks (postterm). It's hard to change how generations of people think/speak, but if I were still practicing medicine, I think I would push for new terminology. Heck, I might anyway. Thanks for this thoughtful question. – anongoodnurse Mar 16 '23 at 15:54
  • So @BryanKrause, it seems we have three different expressions for the same measure. At the end of the 8th week in the gestational timeline, the embryo is 6 weeks old and the woman was actually pregnant for 5 weeks (because for the 30%+ times that the embryo or blastocyst never implants, we don't consider that the woman was ever pregnant). – robert bristow-johnson Mar 16 '23 at 16:40
  • @robertbristow-johnson Even definitions of "pregnant" vary; some would consider fertilization the beginning of pregnancy, others count implantation. Ultimately, whether in legal contexts or medical ones, it's important to be clear about definitions used in any given context, but that doesn't necessarily mean it's possible or desirable to standardize, because different definitions make more sense in different contexts. To a biologist, this is kind of just everyday life and does not seem that unusual. To the general public, it's somewhat mystifying. – Bryan Krause Mar 16 '23 at 16:45
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    If the blastocyst never implants, it's a non-issue. There's no escaping that an implanted blastocyst has been "a potential life" for about 5 days. You can't change that by deciding that those days don't count. – anongoodnurse Mar 17 '23 at 12:16
  • That's fine with me @anongoodnurse . If the blastocyst never implants, we can say the woman was, in fact, pregnant for 5 days but no one ever knew she was. But I still consider it odd to say in one sentence "Gestation is the period of time between conception and birth." and then say "Gestational age is ... measured in weeks, from the first day of the woman's last menstrual cycle to the current date" when two of those weeks the woman was clearly not gestating. – robert bristow-johnson Mar 17 '23 at 16:16
  • Language is funny. "Literally" no longer means literally. In medicine, we ask, "How many weeks... (are you along, etc.)" Or we ask for the due date. Interestingly, when ultrasounds are done to determine how far along a woman is, they report in it weeks corellating to time since LMP. Go figure. – anongoodnurse Mar 17 '23 at 21:47

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