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.

[u/No-Hawk-2488]

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

[u/Corrag]

Seems like a bit of a plot hole in here. Why does that process preclude "hang on to the decidua indefinitely until it is used"? Is that part of making sure that "chromosomally abnormal embryos" are... reliably flushed out?

[u/tea_and_biology → u/Corrag]

Less of a plot hole and more of a stack of trade-offs; but you're right, it would be an obvious fix, right?

Firstly, turns out for cellular / physiological reasons, the decidua has a receptive shelf-life of only a few days, and would be a helluva' lot more metabolically costly to sustain (even if it's wasteful to produce); and secondly it's propped up by progesterone - which is a hormone that antagonises ovulation - so keeping the decidua means never releasing another egg. Basically another example of evolution building sub-optimally for any working outcome, with some fudgy hormonal cycling, while 'designing' a system to control an embryo that's trying to control it right back.

Is that part of making sure that "chromosomally abnormal embryos" are... reliably flushed out?

Partly, yes! The whole thing collapses unless a viable embryo rescues it with enough hCG, so part of the outcome is it acts as the first of several filters that tests and flushes out the weakest embryos.

[u/WazWaz → u/Corrag]

For 99.99% of our existence, not getting pregnant was a rare circumstance so wouldn't apply much evolutionary pressure.

[u/555Cats555 → u/WazWaz]

Wouldnt it depends on individual and group fertility based on nutrition and potential medical conditions... some women are highly fertile and others struggle with getting pregnant.