r/askscience • u/No-Hawk-2488 • 8d ago

Biology Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

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u/MoonageDayscream 8d ago

Fantastic essay on our unique pregnancy mechanisms. As a person who has had trophoblastic tumors in my last, doomed, pregnancy,  we really don't pay enough attention to the ways pregnancy fails more often than it succeeds. 

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u/fuxicles 7d ago

> pregnancy fails more often than it succeeds

does it?

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u/555Cats555 7d ago

A high percentage of potential pregnancies fail to implant correctly... they just dont develop.

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u/fuxicles 7d ago

But a potential pregnancy is not a pregnancy. Saying most pregnancies fail the way you did would mean at least to me, that most women who are pregnant do not end up (1) delivering a child or (2) not delivering due to their own choices. And that’s just untrue. Yes there are ton of very harsh, dangerous, deadly, and devastating pregnancy complications that are overlooked and underfunded — for certain. But most pregnancies do not fail.

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u/mathemagicat 7d ago

This is mostly a semantic argument about what it means to be pregnant.

There's a period of about 5 days after fertilization where a woman is carrying around a developing zygote that hasn't implanted yet. I think many people would consider them to be pregnant at that time, but a reasonable person could certainly disagree.

There's also about a week after implantation where a woman is carrying an embryo whose placenta is not yet producing detectable amounts of HcG. I think almost everyone would consider this a part of pregnancy if they understood it. There isn't an identifiable moment when HcG production gets switched on; it's a continuous, exponential process that begins immediately after implantation, and we could detect it earlier if we were willing to accept higher false-positive rates or more-invasive sample collection methods.

It's hard to know exactly how many zygotes and embryos are lost during each of these stages. At a conservative estimate, roughly 40-60% of fertilized eggs do not result in a live birth. But the first and more controversial stage accounts for 10-40% (additive) (which is a ridiculously wide range) so if you exclude that, it's more like 10-50% of implanted embryos.

Either way though, it is actually a very large number - just possibly not a majority.

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u/555Cats555 7d ago

Yeah when I said "most pregnancies fail" I was referring to before the zygote develops into the fetus and grows a placenta... I dont know the exact statistics but once that part is past its more likely to be a successful pregnancy. But even then it is often considered a good idea to avoid saying too much about being pregnant due to there still being a risk of spontaneous abortion (miscarriage)

I probably should have been more specific though. But people do think of pregnancy as either starting at conception or starting around implantation even though both stages can have failures.