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The menstrual phase begins on Day 1 of the cycle when fertilization does not occur. The corpus luteum deteriorates, causing estrogen and progesterone levels to drop significantly. This hormonal decrease triggers the release of prostaglandins, which initiate the breakdown of the endometrial functional layer and begin menstruation.

Prostaglandins released during the menstrual phase cause uterine spiral arterioles to constrict, cutting off oxygen and nutrients to the stratum functionalis. Deprived tissues break down, and weakened arterial walls eventually rupture. Blood seeps into connective tissues and is shed through the vagina along with tissue fluid, mucus, and epithelial cells.

Menstrual discharge consists of a mixture of blood, tissue fluid, mucus, and epithelial cells shed from the endometrium. This mixture results from the breakdown and shedding of the stratum functionalis, the functional layer of the endometrium. The discharge typically lasts one to five days during the menstrual phase.

After menstruation, the endometrium thins to approximately 2 to 5 millimeters, with only the stratum basalis layer remaining intact. This deeper basal layer serves as the foundation for regenerating a new functional layer during the next menstrual cycle, ensuring the endometrium can support future pregnancy.

The menstrual phase usually lasts one to five days and is considered Day 1 of the menstrual cycle. During this time, the stratum functionalis degenerates and sheds completely. The duration varies among individuals, but this phase marks the beginning of hormonal regulation of the menstrual cycle.

Dysmenorrhea, or painful menstruation, can result from myometrial contractions commonly called cramps, uterine inflammation, or conditions affecting adjacent pelvic structures. While many individuals experience menstruation without significant discomfort, others suffer from these pain-related complications during the menstrual phase of their cycle.

When fertilization does not occur, the corpus luteum deteriorates, causing circulating levels of estrogen and progesterone to decrease dramatically. This hormonal drop is the primary signal that triggers prostaglandin release and initiates the cascade of events leading to endometrial breakdown and menstruation.