Why does menstruation occur?

Menstrual cycle refers to female reproductive cycle characterized by regularly occurring changes in uterine lining and the loss of blood and tissue during this cycle is known as menstruation. The cycle includes egg maturation and preparation of uterus for implantation by thickening of endometrium under hormonal regulation. In absence of egg fertilization, egg rupture is followed by disintegration of uterine lining (endometrium). Since, uterine wall is rich in blood capillaries and tissues, it’s shedding off results in release of blood and tissues which come out of vagina in menses.