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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.

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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 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 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 at 4:22
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    @robertbristow-johnson Furthermore, implantation doesn't occur until ~a week later.
    – Bryan Krause ♦
    Mar 16 at 14:13
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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 at 12:16

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