Figure 3.Hormonal levels through the menstrual cycle. The cycle is driven by alterations in GnRH pulsation, which differentially favor the production of gonadotropins. Follicular development fostered by FSH results in E2 production, which, in turn, causes positive feedback to produce the LH surge. After ovulation, the corpus luteum produces P, causing a slowing of the GnRH pulse generator and decrease in LH. The arrows indicate GnRH pulsations. The length of the arrow indicates the amount of GnRH secreted. The distance between arrows indicates pulse intervals. (From Marshall JC et al: Gonadotropin-releasing hormone pulses: Regulators of gonadotropin synthesis and ovulatory cycles. Recent Prog Horm Res 47:155-189, 1991).