In the “positive” view, menstruation has a serious biological function. Every month, at least in humans, the lining of the uterus grows thick, supposedly to provide a soft cushion for any fertilized eggs that find their way into it. If no embryo implants, the uterus sheds this lining, if only because it would be costly, in a caloric sense, to maintain it—hence the mess of blood and tissue fragments that make up the menstrual discharge. But repeated monthly over decades, the shedding of the uterine lining is itself very costly; women typically lose a pint of blood a year and sometimes several
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