A new interpretation of human menstruation is presented, resulting from a cross‐disciplinary investigation of evolution, developmental biology, and physiology. A process evolutionarily associated with childbirth expresses itself as menstruation in women for whom frequent and continual failure to conceive has become the default situation. In humans and Old World primates, contractile uterine spiral arterioles evolved as the complement of the highly invasive hemochorionic placenta and is the selected phenotype. Placental progesterone withdrawal during the last stage of birth leads to arrested (...) blood flow through maternal spiral arterioles, allowing detachment of the deciduous placenta with minimal maternal hemorrhage. In nonpregnant females, progesterone withdrawal from a degenerating corpus luteum initiates menstruation and stops blood flow through uterine spiral arterioles. Both events share similar physiological mechanisms and sequences. This explanation may improve our understanding of a recurrent event experienced by half of the human population and for a quarter of their adult reproductive life. (shrink)