In a video showing the first ever movie of an embryo (blastocyst?) forcibly digging its way into a (lab grown) endometrial lining in an attempt to implant, a commenter asked how it was energy efficient to bleed every month rather than reabsorb the uterine lining.
Well
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Here’s what I felt compelled to come up with. It’s not maximizing for energy/nutrient protection, it’s minimizing the danger from infection
The menstrual tissue being discarded is subject to outside contamination. Shedding it keeps the spread of contamination to a minimum. The fallopian tubes open to the abdominal cavity, they aren’t in a closed loop with the ovaries (this is why ectopic pregnancy can occur in strange places). And in the case of a failed implantation, shedding expels it from the body - remember half its DNA is foreign!
Without the placenta (and other pregnancy specific elements) running interference, the mom’s immune system will damage or kill the blastocyst/embryo. Then there’s dead, potentially necrotic tissue with foreign DNA in a place open to the outside world but also into the gut. Best not to risk it getting into the gut. It’s best not to reabsorb that if you don’t have to. I imagine those that reabsorbed the tissue in our evolutionary antiquity fared less well over millions of years so now humans are stuck with bleeding.
Although historically, a great number of women only had a few years worth of periods in total after their first kid. The majority spent the majority of their post pubescent life pregnant or recovering from pregnancy. Unless subject to the Rh factor, other fertility issues or they managed to avoid sex, they had a baby, recovered somewhat, maybe had a period, maybe not, then got pregnant again.
Regarding contamination - anything going in the vag can catch a ride on a sperm or be pushed by a dick or dildo being slammed into it , on a feminine hygiene product or randomly move around into the cervix. This could be sperm, ejaculate, crusty unwashed urine, dirt, sand, oil, lube, bits of unsafe toys, lake water, you name it. All this foreign tissue is an infection risk.
(iirc, the hymen is there to keep literal and figurative crap out of the cervix and uterus when we’re babies who wear diapers and can’t stand up for a gravity assist.)
Mucosal membranes are where we have our greatest risk of infection because it’s purpose is to interact with external bodies and only let the ones we need in, but it’s not as good as intact epidermis/skin because it does need to let some stuff in, like water or sperm or placentas. the cervix is kinda like the end of a balloon with a tight neck. It blocks a lot, but it has a hatch that can be breached. And wily enough germs or forcefully introduced grit, can get into the uterus, which is lined with yet another semi-permeable surface. Because we don’t want stuff going up the fallopian tubes easily, the forces acting on the area favor flushing detritus out. When the uterine lining detaches, it too gets flushed out.
QED, it makes sense for people with a uterus to bleed regularly rather than risk absorbing dangerous contaminants and infectious pathogens
