The principle of menstruation is that the endometrium undergoes cyclic proliferation and exfoliation under the influence of the ovaries’ cyclic secretion of estrogen and progesterone, resulting in bleeding. The ovaries are regulated by the hypothalamus and the pituitary gland and secrete relatively large amounts of estrogen, which is accompanied by the development of follicles. Estrogen promotes the endometrium to proliferate and when it reaches a certain thickness, the follicles of the ovaries also mature. The ovaries, in turn, influence the hypothalamus and pituitary gland, causing a change in the role of the hypothalamus and pituitary gland in relation to the ovaries, and a decrease in the ovary’s ability to secrete estrogen and the start of the secretion of progesterone. Progesterone causes the lining of the uterus to change from a proliferative phase to a secretory phase that lasts until the next menstrual period. If an egg is fertilized to form a fertilized ovum, the fertilized ovum will not have a menstrual period after it has developed after it has been deposited. If no fertilized egg is formed or if the fertilized egg does not successfully attach to the uterus, the level of progesterone drops rapidly, causing the endometrium in the secretory phase to peel off and bleed, which results in menstruation, and so on and so forth is the menstrual cycle.