The ovary produces a mature egg which is called as an ovum. The egg is produced once in a month. When the egg is released and ready for fertilization, the cell lining of the uterus thickens for the implantation of the fertilized egg. If the ovum gets fertilized, the zygote gets implanted. However, if fertilization does not occur, the egg withers away and the thick cell lining of the uterus gets removed and discharges the contents through the birth canal. This process is referred to as menstruation.