31.23 : Menses Phase

The uterine cycle begins with the menstrual phase, which is considered day one of the cycle and typically lasts about five days. This phase is characterized by the degeneration and shedding of the stratum functionalis, the functional layer of the endometrium.

When fertilization does not occur, the corpus luteum deteriorates, causing a significant drop in the levels of estrogen and progesterone in the body. This hormonal decrease triggers the release of prostaglandins, which cause the uterine spiral arterioles, small branches of arteries, to constrict.

This constriction results in the stratum functionalis being deprived of oxygen and nutrients. The deprivation leads to the breakdown of secretory glands and other tissues within the stratum functionalis. As deprivation continues, the arterial walls weaken and rupture, allowing blood to seep into the connective tissues of the functional layer.

This process culminates in menstruation, during which blood, tissue fluid, mucus, and epithelial cells are expelled through the vagina. The endometrium thins to approximately 2–5 mm, leaving only the stratum basalis intact to regenerate the next cycle's functional layer.

While many experience menstruation without significant discomfort, some women suffer from dysmenorrhea or painful menstruation. This pain can result from myometrial contractions ("cramps"), uterine inflammation, or conditions affecting adjacent pelvic structures.