Comment by derefr | Hacker News Reader

Comment by derefr | Hacker News Reader

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tl;dr:

What we know for sure: foetuses produce hormones to keep the endometrial tissue from thinning. Once a zygote has implanted, the mother's hormones are thus no longer in control of whether her uterus will support its growth or not. Menstruation, when it happens, forcefully cleans out implanted zygotes, returning the uterine hormonal balance to normal.

What we guess that that implies: menstruation is a side-effect of enabling spontaneous abortion (miscarriage) of a still-live foetus, thus allowing females of those species to not have to go through the "avoid males you're not interested in during estrus" dance that most animals do. This confers social advantages.

In humans specifically, estrus is hidden completely from both male and female awareness. This confers further social advantages, but only could have evolved to fixation if the sexual-selection advantage of applying a "proximity during estrus" filter was completely outmoded/obviated by miscarriage.

The problem with that theory is I don't know of any women who are actually able to miscarry at will.

A garbled version of that theory got some politicians in hot water with the whole "during rape, their body can shut it down" comment.

It's not in the control of the conscious mind. It is controlled by the body (or, you might say, by an evolutionary adaptation instantiated in the linkages of hormonal pathways in the brain.)

Note that miscarriage and spontaneous abortion are separate things. Miscarriage happens because the foetus dies in the womb, or the womb is ruptured, and the mother's body detects this. Any other cause is spontaneous abortion—effectively "something the mother['s body] did."

In humans, at least the experience of the emotions of extreme stress, and extreme fear, can cause spontaneous abortion. In a number of other animals (with a large subset of those being the ones that menstruate), there is the Bruce effect[1]: when the detection by the vomeronasal organ of an unfamiliar male's pheromonal signature precedes/causes spontaneous abortion.

[1] http://en.wikipedia.org/wiki/Bruce_effect

That is a poor explanation given that extreme stress is known to have all sorts of physiological effects and is noted to negatively effect reproductive success in numerous species. But not all those species menstruate as noted by the article.

The thing muddying the water with stress, is that most things that trigger stress are also bad for the body, and thus for foetal development. Making rats run until they collapse from exhaustion might cause miscarriage, but it's unclear whether that is due to stress-hormone-induced spontaneous abortion, or due to the mother's body using up some micronutrient necessary for the foetus, thus causing a developmental defect which either kills the foetus or stalls its metabolism enough to be detected.

I'm unaware of whether there has been the study of the effects of purely psychological (or synthetic, using injected epinephrine) stress on spontaneous abortion rates.

It's much clearer with fear, though; things that cause a fear response don't tend to harm you (in fact, in an adaptive environment, to the degree that a fear response is created, you tend to avoid harm you would otherwise succumb to) but do trigger spontaneous abortion anyway. This would be much simpler to prove/disprove a purely-hormonal effect.

It doesn't have to be "at will", our bodies do a lot of processing without us being consciously aware of it. I think it is well within reason that the body may detect an abnormal embryo and respond to it.

Agree. I've seen ability to orgasm linked to selectivity for fertilization, as the speculation (I don't follow it enough to know if it is a theory or not in any allowable sense of the word) is that (female) orgasm during intercourse helps the sperm reach the egg.

>What we guess that that implies:

No.

This is not a tl;dr of the article, which proposes several scientific hypotheses, which do not include what you have said.

It mentions the case of spontaneous abortion being beneficial where the embryo is not likely to be viable or the foetus is causing harm to the mother.

There were a few explanations proposed. This was one of them, but it was unclear if this is the theory with the most support.

I suspect it's more a matter of ejecting poorly implanted zygotes—implanted enough to get some nutrients, but not enough to overtake the mother's hormonal balance. A poor implant could grow for a period of time before ultimately failing, fomenting a much more dangerous (for the mother) situation later in the term.

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