Reproductive aging is associated with an acceleration in the decline of ovarian follicle numbers. Menstrual cycles change from regular to irregular when follicle numbers reach a critical level of between 100 and 1000. With the onset of irregular cycles, there is an increased incidence of anovulatory cycles and elongated ovulatory cycles, both of which are characterized by high FSH, low estradiol, low inhibin B, and, in the case of elongated ovulatory cycles, low progesterone. During normal length ovulatory cycles, changes in gonadotropins, sex steroids, and the inhibins remain subtle and reflect preservation of normal menstrual cycle function. These normal cycles occur less often as the final menstrual period approaches, and high FSH levels, low estradiol, and low inhibin levels predominate.