The Menstrual Cycle

The menstrual cycle is a sequence of monthly changes in the endometrium.

The menstrual cycle may last between 21 and 38 days and is typically divided into the menstrual phase lasting days 1 to 5, the proliferative phase lasting days 6 to 14, and the secretory phase lasting days 15 to 28.

In the menstrual phase, triggered by the declining estrogen and progesterone levels, the uterine endometrium sheds its stratum functionalis layer.

This shedding process, called menstruation, results in uterine bleeding. The bloody discharge is a mixture of blood, tissue fluid, mucus, and epithelial cells.

During the proliferative phase, the endometrium rebuilds itself under the influence of estrogen released by the growing ovarian follicles. Ovulation occurs at the end of this phase.

The secretory phase prepares the endometrium for potential embryo implantation in response to progesterone and estrogen from the corpus luteum.

If fertilization does not occur, the corpus luteum degenerates, causing progesterone and estrogen levels to fall, leading to menstruation.