What changes are observed in the uterus if fertilisation does not occur?

Explanation

If the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg Its lining becomes thick and spongy.
This woud be required for nourishing the embryo if fertilisation had taken place. Now however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days.