The menstrual cycle is a cyclic process in which the ovaries and endometrium of the uterus undergo changes in response to the release of hormones at both the hypothalamic-hypophysial and ovarian levels.
As for the endometrium, it is shed at the beginning of each menstrual cycle if there has been no pregnancy. This is what causes the vaginal bleeding characteristic of menstruation.
From this moment on, the secretion of FSH, LH and estrogens in the first place causes a new endometrium to begin to proliferate and increase in thickness. Subsequently, with the synthesis of progesterone, the endometrium enters the secretory phase and undergoes changes that make it receptive to embryo implantation.
When estrogen and progesterone levels drop again at the end of the menstrual cycle if there has been no pregnancy, the endometrium is shed again with menstruation.