Menstruation is the cyclical shedding of the endometrium triggered by falling progesterone levels. Menstruation is a rare trait found in less than 2% of mammals and likely evolved independently at least 4 times. Why do some mammals menstruate while most do not? The leading hypothesis is that menstruation occurs as a nonadaptive consequence of spontaneous decidualization of the endometrium, which evolved to increase biosensoring of embryo quality. While the trait of spontaneous decidualization (and as a byproduct, menstruation) was likely shaped by natural selection, menstruation also disposes women to conditions such as endometriosis, pre-menstrual syndrome, and bleeding disorders, especially in the contemporary context. (#40693)