Why does menstruation occur?

Menstruation is a process of regular discharge of blood and mucosal tissues that occurs monthly in females.

It occurs because the ovary produces one egg every month, so simultaneously the walls of the uterus thicken, get supplied by blood and the uterus is prepared to receive and nourish the egg if fertilized by the sperm. However, if the egg does not get fertilized by the sperm then this inner wall of the uterus which thickened earlier to receive the fertilized egg, breaks down gradually and is released externally from the vagina in form of blood and mucosal tissues.