31.25 : Secretory Phase

The secretory phase of the menstrual cycle, spanning from day 14 to 28 in a typical 28-day cycle, is a period of significant physiological changes in the female reproductive system. This phase commences immediately after ovulation and is characterized by the preparation of the endometrium for potential embryo implantation.

Following ovulation, the corpus luteum, a temporary endocrine structure, produces progesterone and estrogens. These hormones stimulate the growth and coiling of endometrial glands, enhance vascularization of the superficial endometrium, and increase endometrial thickness to 12–18 mm. Simultaneously, the endometrial glands secrete glycogen and other nutrients into the uterine cavity, serving as sustenance for a potential embryo before implantation.

Rising progesterone levels during this phase cause cervical mucus to thicken, forming a plug as a barrier against pathogens and other foreign materials. These preparatory changes peak approximately one week after ovulation, aligning with the expected arrival of a fertilized egg in the uterine cavity.

If fertilization does not occur, the corpus luteum degenerates during the late secretory phase due to declining levels of luteinizing hormone (LH). This hormonal withdrawal leads to a marked reduction in progesterone, depriving the endometrium of hormonal support. The spiral arteries undergo vasoconstriction and spasms, causing ischemia and deprivation of oxygen and nutrients to the functional layer of the endometrium. Consequently, endometrial cells undergo apoptosis, and the glands regress, initiating menstruation on day 28 and starting a new uterine cycle.