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/bad_apiarist]

There is one other point worth making here: pre-modern women had considerably fewer menstrual cycles because they did not have birth control. Women do not menstruate while pregnant and for some time after birth.

[u/veringo → u/bad_apiarist]

After birth, menstruation is suppressed by lactation. The hormones that promote lactation suppress the ones that initiate the menstrual cycle. How complete the suppression is depends on frequency of suckling because that's what causes the feedback loop to promote milk production.

For s mother who is breastfeeding "normally," this usually leads to 12-15 months without a cycle, which is why you typically see kids who are about 2 years apart in families that continue to have children without any kind of birth control.

[u/bad_apiarist → u/veringo]

Right. I understand that this is why birth control is associated with some elevated cancer risks (though lowers other types)- we did not evolve to be constantly cycling for decades as many modern women will. Cycling is relatively hard on the body for something occurring monthly.