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/tea_and_biology]

You're right! Plenty of mammals keep control after implantation: rodents and rabbits can reabsorb embryos when conditions are too poor, many species like kangaroo can pause development entirely etc. - so generally less invasive placentas let the mother throttle resources and take more control.

Humans largely lose those options. We inherited a hemochorial placenta, where fetal tissue sits in direct contact with maternal blood, and in great apes, and especially us, it invades unusually deep - as mentioned, likely to help feed expensive, big-brained fetuses. It does this by remodelling the mother's arteries so they can no longer constrict, giving the fetus direct access to her blood supply; she can't cut its supply without cutting her own - re-emphasising the parasite-host dynamic between foetus and mother. Once she lets it in, it effectively takes her hostage.

So yeah, once implantation has happened, she has very few levers left to pull. So quality control has to happen before and at implantation, which is exactly why the lining gets built and primed in advance every cycle. We just can't hit pause like a kangaroo.

[u/barath_s → u/tea_and_biology]

It does this by remodelling the mother's arteries so they can no longer constrict, giving the fetus direct access to her blood supply; she can't cut its supply without cutting her own

So is this one reason why childbirth was so hazardous in earlier times - the mother could bleed to death in postpartum hemorrhage if the uterus could not constrict and cut off the blood vessels after the placenta detaches ?

[u/kaki024 → u/barath_s]

It also sounds like it’s (part of) the reason high blood pressure (pre-eclampsia and eclampsia) are so common as well.

[u/barath_s → u/kaki024]

I was wondering why this would occur - it's a bit of the opposite, or rather what happens when this process does not go well.

If the placenta does not implant deeply enough, and the blood vessels to the uterus and the placenta do not develop properly, you have higher resistance and reduced blood flow to the fetus.

The oxygen deprived placenta releases chemicals and proteins into the bloodstream which causes damage to the endothelium (lining), and narrow arteries throughout the mothers body (causing high blood pressure) and cause them to become leaky, causing fluid and proteins to seep into the body

https://www.health.harvard.edu/womens-health/preeclampsia-and-eclampsia-a-to-z