Hormonal Regulation of the Menstrual Cycle

Gonadotrophin-releasing hormone (GnRH) initiates the menstrual cycle through pulsatile release from the hypothalamus. At puberty, GnRH begins a monthly release pattern that stimulates the anterior pituitary to secrete follicle-stimulating hormone and luteinizing hormone. The frequency and amplitude of GnRH pulses vary throughout the cycle, with faster pulses favoring LH release and slower pulses favoring FSH release, thereby coordinating ovarian and endometrial changes.

Rising estrogen levels during the follicular phase trigger a positive feedback loop that enhances GnRH secretion from the hypothalamus. This increased GnRH stimulation causes a surge in luteinizing hormone, which ultimately leads to ovulation. Additionally, estrogen causes the endometrium to thicken and vascularize, preparing the uterus for potential pregnancy.

Following ovulation, high luteinizing hormone levels promote corpus luteum formation from the ruptured follicle. The corpus luteum predominantly secretes progesterone with moderate estrogen, preparing the uterus for potential pregnancy. If fertilization occurs, human chorionic gonadotropin from the embryo maintains the corpus luteum and endometrium. Without fertilization, the corpus luteum degenerates, causing hormone levels to drop and triggering menstruation.

Progesterone, secreted by the corpus luteum after ovulation, further enhances the endometrium beyond the thickening and vascularization initiated by estrogen. This enhanced endometrial state creates an optimal environment for embryo implantation and pregnancy maintenance. If fertilization does not occur, the decline in progesterone levels removes negative feedback on the hypothalamus, allowing the cycle to restart.

Without fertilization, the corpus luteum degenerates, causing estrogen and progesterone levels to drop sharply. This hormonal decline removes negative feedback on the hypothalamus, allowing gonadotrophin-releasing hormone pulsatility to resume. The renewed GnRH pulses stimulate the anterior pituitary to secrete follicle-stimulating hormone, initiating follicle development and beginning a new menstrual cycle.

Follicle-stimulating hormone stimulates granulosa cells of ovarian follicles to proliferate and produce estrogen through aromatization of androgens. Luteinizing hormone supplies these androgens by stimulating theca cells. This coordinated action of FSH and LH increases estrogen secretion, which thickens and vascularizes the endometrium while triggering the positive feedback that eventually causes the LH surge leading to ovulation.

If fertilization occurs, the developing embryo secretes human chorionic gonadotropin (hCG), which maintains the corpus luteum and prevents its degeneration. Sustained corpus luteum function preserves elevated progesterone and estrogen levels, which maintain the endometrium. This hormonal support prevents the endometrial shedding that normally occurs during menstruation, allowing pregnancy to continue.