Why does menstruation occur?
Every month, the ovary releases one egg. The uterus prepares itself to receive a fertilised egg by making its lining thick and spongy. This lining is richly supplied with blood and would be required for nourishing the embryo if fertilisation had taken place.
If the egg is not fertilised, it lives for about one day. Since the thick lining is no longer needed to support a growing embryo, it is not required any longer.
Consequently, the lining slowly breaks down 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.
Explanation
The answer is derived from Section 7.3.3 (c) of the textbook context. It explains the biological process where the uterus prepares for potential pregnancy, and when fertilisation does not occur, the prepared lining is shed. The key points included are the monthly release of the egg, the preparation of the uterine lining for nourishment, and the shedding of this lining as menstruation when the egg remains unfertilised.