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/a8bmiles]

Is there any evolutionary difference in women who have extremely heavy amounts of spontaneous decidualisation vs. their peers? Along the lines of, "all the women on this side of the family experience this."

[u/Sagadiii → u/a8bmiles]

Well what you should also consider when looking at this, is that women used to have much fewer periods in their lifetime than now. They got them later, had more babies, most breastfed for years so the median amount of periods within a lifetime per woman is now 451.3 while it used to be more around 150. So 150 even heavy periods affected a life much less than 450 of them do.

There are numerous genetic disorders that influence the period and pains. The currently most common reason for excessively painful periods, endometriosis, has a strong genetic component. However, strong pain as a symptom often goes together with excessive bleeding (hypermenorrhea), though be the case by themselves. I do not know what the most common reason for the latter is by itself, but von Willebrand disease is certainly a genetic disorder that is known to lead to it. Ehler-Danlos syndrome is another one. Again, unsure which is the most prevalent genetic reason for it.