Breast-feeding through the suckling stimulus suppresses fertility for a variable time after birth. Initially there is a period of pituitary gonadotroph recovery from the suppressive effects of the high steroid levels of pregnancy, followed by a period of suppressed ovarian activity associated with limited follicle growth. During this period of breast-feeding-induced amenorrhea, the pulsatile secretion of luteinizing hormone (LH), which reflects hypothalamic gonadotropin-releasing hormone (GnRH) release, is erratic and much slower than the one pulse per hour required in the normal follicular phase of the menstrual cycle to drive follicle growth. At some time the suckling stimulus drops below a threshold resulting in a resumption of reasonably organized pulsatile LH secretion, which is associated with development of follicles and some steroid secretion. However, positive feedback of estradiol which triggers the preovulatory LH surge and ovulation appears to be blocked by continued suckling, until suckling is reduced further and positive feedback and ovulation resumes. Often while women continue to breast-feed the first few ovulations and menses are associated with inadequate corpus luteum function, which would probably not support a pregnancy. Eventually normal menstrual cycles resume when suckling declines further. The duration of amenorrhea and subsequent period of inadequate luteal function varies greatly between mother-baby combinations, and in different societies. Exactly how the suckling stimulus reduces pulsatile secretion of GnRH/LH is not clear, although clinical studies do not support a role for opioids or dopamine. The role of prolactin remains uncertain since suckling releases both prolactin and suppresses GnRH release. Regardless of the precise mechanism, it is clear that breast-feeding in women can suppress fertility for prolonged periods, and women may proceed from pregnancy through lactation to another pregnancy and lactation with no menstrual period for several years.