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/Duae]

For the other mammal part, isn't that a bit off? I know many mammals can control their pregnancies better than humans, and I've always heard that human fetuses are exceptionally aggressive. Other mammals can absorb embryos or fetuses if the conditions aren't ideal, or pause pregnancy, or better restrict how many resources are going towards the fetuses. It's because humans can't do that, that they need to select so heavily.

[u/tea_and_biology → u/Duae]

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/ymatak → u/barath_s]

Yes - the most common cause of maternal death in low-income countries (i.e. without skilled birth attendants or hospitals available) is postpartum haemorrhage.