is a cycle which occurs in most primates, including humans, during which the average menstrual cycle will last 28 days. Hormones are typically the driver and mechanism of menstrual cycles, in humans, this is concentrations of certain gonadotropins and other hormones such as FSH (otherwise known as follicle stimulating hormone) and LF (luteinizing hormone).

MENSTRUAL CYCLE: "The average menstrual cycle in a human being will last 28 days and during which the individual is most likely to reproduce."