The menstrual cycle is a recurrent sequence of changes in the uterine endometrium, specifically its functional layer, the stratum functionalis. This cycle prepares the uterus for potential pregnancy. This cycle typically spans 21–35 days, averaging 28 days, and aligns with the ovarian cycle, regulated by fluctuating levels of ovarian hormones, primarily estrogen and progesterone.

The menstrual phase occurs from days 1 to 5 and involves the shedding of the stratum functionalis, as a uterine discharge composed of blood, tissue fluid, mucus, and epithelial cells. This process is triggered by the constriction of uterine spiral arterioles due to declining levels of progesterone and estrogens, resulting in oxygen deprivation, cell death, and detachment of the functional layer. During this phase, the endometrium thins, leaving only the stratum basalis which remains to regenerate the next cycle's functional layer.

From days 6 to 14, the proliferative phase begins. Estrogens released by growing ovarian follicles stimulate the endometrium to rebuild itself. The endometrial thickness doubles as cells in the stratum basalis proliferate to form a new stratum functionalis. This phase ends with ovulation, driven by a luteinizing hormone (LH) surge from the anterior pituitary.

The final phase, known as the secretory phase, occurs from days 15 to 28. The endometrium prepares for potential embryo implantation during this period, with progesterone and estrogens from the corpus luteum promoting growth and coiling of the endometrial glands, vascularization of the superficial endometrium, and thickening of the endometrium. If fertilization does not occur, the corpus luteum degenerates, causing progesterone and estrogen levels to drop. This leads to the regression of glands, ischemic death of endometrial cells, and ultimately, the onset of menstruation, restarting the cycle on day 1.