Menstrual cycle

The menstrual cycle is driven by the hypothalamic-pituitary-ovarian axis. The hypothalamus releases GnRH in pulses, prompting the anterior pituitary to secrete FSH and LH. These gonadotropins act on the ovary, and the ovarian steroids they stimulate — estrogen and progesterone — in turn act on the uterine endometrium and feed back on the pituitary.

During the follicular phase, FSH drives growth of ovarian follicles, whose granulosa cells secrete rising estrogen. Estrogen thickens the endometrium (the proliferative phase) and, once it is sustained at a high level, switches from negative to positive feedback and triggers an LH surge. That surge causes ovulation, releasing the oocyte around day 14 of an idealized 28-day cycle. The ruptured follicle becomes the corpus luteum, which secretes progesterone throughout the luteal phase and converts the endometrium into a secretory, glandular lining ready to support implantation.

If fertilization does not occur, the corpus luteum regresses after roughly 14 days. Progesterone and estrogen fall sharply, the endometrial lining is shed as menses, and the loss of negative feedback allows FSH to rise and begin the next cycle. If fertilization does occur, hCG from the developing embryo rescues the corpus luteum so progesterone stays elevated and the lining is maintained. The luteal phase is relatively fixed in length, so variation in total cycle length comes mostly from the follicular phase.

USMLE Step 1 tests the hormonal sequence directly, including the feedback switch that produces the LH surge and the relationship between the cycle and menopause. NCCT and NHA medical assistant exams such as the CCMA cover the cycle in the context of assisting with obstetrics and gynecology, and the NPTE covers reproductive physiology within its other-systems content.