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.
The secretory phase of the menstrual cycle starts after ovulation and spans from day 14 to day 28 in a typical 28-day cycle.
During this period, triggered by progesterone and estrogen produced by the corpus luteum, the endometrium gears up for possible embryo implantation.
As a result, the endometrium thickens to about 12-18 mm.
The endometrial glands secrete nutrients like glycogen into the uterine cavity, which helps sustain the embryo until it implants into the uterine lining.
Additionally, the cervical mucus thickens, forming a cervical plug that blocks the entry of sperm, pathogens, or other foreign materials.
Approximately one week after ovulation, these preparatory changes reach their climax, coinciding with the time a fertilized egg might arrive in the uterus.
If fertilization does not occur, the corpus luteum degenerates during the late secretory phase due to declining levels of luteinizing hormone.
The resulting fall in the progesterone and estrogen levels sets the stage for menstruation to begin on day 28, restarting the cycle.
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