During the menstrual phase, if the ovum is not fertilized, the corpus luteum regresses from the 24th day of the cycle.
This will cause a decrease in estrogen and progesterone levels.
It will result in shrinkage of endometrium, coiling, and spasm of arteries leading to focal necrosis of the endometrium.
The necrosed endometrium is shed along with blood and other secretion.
The follicular phase is a proliferative phase or preovulatory phase.
Hence, the menstrual phase is followed by a follicular phase.
During this phase, another follicle containing the egg released by the ovary will grow in size, accumulate nutritional cell aggregation, and mature to form the Graafian follicle.
In case it is fertilized, it will implant in the uterus.
In case of no fertilization, the menstrual cycle will repeat.