Menstruation

Menstruation is both under- and overutilized in our understanding of human reproduction. The most popular hypothesis as to why menstruation evolved has been that it is a byproduct, a “non-adaptive consequence” of the terminally differentiating endometrium. With that label, menstruation has been underexplored as being evolutionarily or mechanistically meaningful. However, more than a non-adaptive consequence, we argue that menstruation is a part of the adaptive complex of the entire menstrual cycle. New evidence suggests the process of menstruation and menstrual effluent are physiologically relevant to processes of implantation and endometrial repair. Conversely, we argue that the literature overemphasizes the timing of first menstruation (menarche) as an indicator of negative childhood experiences or a harbinger of later health concerns. Age at menarche varies due to stressors, but these relationships are not universal. Menarche also varies due to supportive variables. Thus, scholars often overestimate the power of age at menarche to inform our understanding of reproductive functioning and life history, while also using it as a tool to pathologize culturally specific notions of being “early” or “late.” We advance the perspective that menstruation is physiologically meaningful and has an evolutionary basis, where menarcheal timing is less physiologically meaningful than other pubertal milestones.