Final answer:
Menstruation is the B. monthly shedding of the uterine lining when a mature egg is not fertilized. It marks the beginning of the menstrual cycle, which is regulated by hormones and is essential for reproduction.
Step-by-step explanation:
Menstruation Definition
Menstruation is the monthly shedding of tissue from the lining of the uterus, called the endometrium. This process is part of the broader menstrual cycle, which prepares the female body for potential pregnancy. Menstruation occurs when a mature ovum, or egg, is not fertilized; the built-up endometrial lining is not needed for a pregnancy and is therefore shed. This is synonymous with the monthly period or menses.
Menstrual Cycle Phases
The menstrual cycle can be divided into phases. The first phase is the menstrual phase where shedding occurs. It is followed by the proliferative phase, where the endometrium rebuilds itself, and if pregnancy doesn't take place, the final phase, called the secretory phase, ends with another menstrual period. All these changes are regulated by hormones from the hypothalamus, pituitary gland, and ovaries.