Age-related Changes in Ovarian Follicular Wave Dynamics

Inclusion Criteria: * Informed consent obtained * Age of 18-55 * 18\<BMI\<35 * Menstrual cycle history: * MRA group: regular cycles * LRA-1 group: history of regular cycles, with variable cycles allowed (no more than 3 months amenorrhea) * LRA-2 group: variable cycles allowed (no more than 3 months amenorrhea) * Has not taken hormonal contraception within 2 months of initiating study procedures * Agrees to use back-up contraception throughout the course of the study, if sexually active (condoms to be provided) * 3.0 \< Prolactin \< 20.0 ug/L * 0.30 \< TSH \< 5.50 mIU/L * Normal complete blood count (CBC) * Beta human Chorionic Gonadotropin \< 5IU/L. Exclusion Criteria: * Smokers * Documented ovarian failure * History of or currently-diagnosed infertility of unexplained or female origin; * Medical conditions known or suspected to interfere with reproductive function (including PCOS) * Presence of only one ovary * Ovaries inaccessible with transvaginal ultrasonography * The use of any medications known or suspected to interfere with reproductive function * Ongoing pregnancy * Lactation * Participation in an investigational drug trial in the 30 days prior to the pre-study visit * Planning to undergo surgery during the course of the study.