why does the linning of uterus become thick and spongy every month?

Every month before ovulation the lining of uterus become thick and spongy due to the action of progesterone to provide a protective lining for the development of embryo after fertilization. Every month the body gets prepared in this way for the presumptive embryo. If fertilization does not occur, all the materials of lining wash away from the body through menstruation.