Answer:
In females, one ovum matures and is released by one of the ovaries once every 28 - 30 days. During this period, the wall of the uterus becomes thick so as to receive the fertilized egg. The fertilized egg embeds itself into the uterine lining. This event results in pregnancy. [1 Mark]
If in case, fertilization does not occur, the released egg and the thickened lining of the uterus along with its blood vessels are shed off. This causes bleeding in women which is thus, called menstruation. [1 Mark]