What event marks the end of one menstrual cycle and the beginning of another?

Menstruation marks the end of one menstrual cycle and the beginning of another.

Explanation:

The period between the first day of a period and the first of the next period is called the menstrual cycle. That is, menstruation is the event that marks the end of one menstrual cycle and the beginning of another. The menstrual cycle is usually 28 days, but it is considered normal to last between 21 and 35 days. During the menstrual cycle, the body prepares for a possible pregnancy.

To understand what the menstrual cycle is, it is important that you know that in the first two or three years of the period it is normal for the cycles to be a little irregular. After this time, the cycles usually become more regular and, generally, after the first delivery, the cycles tend to stabilize until reaching 40-45 years. From this age on, the cycles will become irregular again before the onset of menopause.

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this is really easy just look up what these words mean