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mensuration is the process of shedding of the uterine lining on a regular monthly basis  .it begins at puberty and is the reproductive cycle of the female body . every month the uterus prepares itself to receive a fertilized egg . therefore the , the inner lining becomes thick and is supplied with the blood to nourish the embryo .if the egg is not fertilized then the lining of the uterus breaks down and get released in the form of blood through the vagina .this lasts for about two to eight days .this cycle occurs every month and is known as menstrual cycle. 
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In the females the limits of reproductive life are very sharp. It begins with the onset of puberty at at the age of of 12-13 years. The appearance of the first menstruation is called menarche. It causes with menopause (cessation of reproduction), at the age of 45-50 years. The recurring ovarian cycles of about 28 days in the reproductive life of a female, accompanied by the release of usually one mature egg, capable of fertilization, are called menstrual cycles in menstruation.