Secretory Phase

The secretory phase occurs from day 14 to day 28 in a typical 28-day menstrual cycle, beginning immediately after ovulation. During this period, the corpus luteum produces progesterone and estrogen, which trigger significant changes in the endometrium to prepare for potential embryo implantation. These hormonal changes peak approximately one week after ovulation, coinciding with when a fertilized egg would arrive in the uterus.

During the secretory phase, progesterone and estrogen stimulate the endometrium to thicken to 12-18 millimeters and increase vascularization of the superficial layer. The endometrial glands coil and secrete glycogen and other nutrients into the uterine cavity to sustain a potential embryo before implantation. These structural and biochemical changes create an optimal environment for embryo development.

Rising progesterone levels during the secretory phase cause cervical mucus to thicken and form a cervical plug that acts as a barrier. This plug blocks the entry of sperm, pathogens, and other foreign materials into the uterus, providing protection during the period when the endometrium is preparing for implantation.

If fertilization does not occur, declining levels of luteinizing hormone cause the corpus luteum to degenerate during the late secretory phase. This hormonal withdrawal leads to a marked reduction in progesterone and estrogen levels, depriving the endometrium of hormonal support and triggering the onset of menstruation on day 28.

When progesterone levels fall due to corpus luteum degeneration, the spiral arteries in the endometrium undergo vasoconstriction and spasms, causing ischemia and oxygen deprivation to the functional layer. This triggers apoptosis in endometrial cells, gland regression, and the shedding of the endometrial lining, initiating menstruation.

Progesterone and estrogen produced by the corpus luteum are the primary hormones responsible for secretory phase changes. These hormones stimulate endometrial thickening, gland coiling and secretion of nutrients, increased vascularization, and cervical mucus thickening. Together, they prepare the uterus for potential embryo implantation.

The secretory phase preparatory changes peak approximately one week after ovulation, which corresponds to when a fertilized egg would naturally arrive in the uterine cavity. This precise timing ensures the endometrium reaches optimal thickness, nutrient secretion, and vascularization exactly when an embryo needs these conditions for successful implantation and early development.