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Is there any explanation or evolutionary advantage to the endometrium constantly cycling between the proliferative and menstrual cycle? Wouldn't it be more advantageous if it kept always fully thickened? I imagine it is less energetically expensive than to be always producing and losing all those cells, blood vessels, etc.

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One hypothesis is to protect the uterus and oviducts from sperm-borne pathogens.

Also Beverly Strassmann has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).

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