Which event marks the beginning of a new menstrual cycle?

The beginning of a new menstrual cycle is marked by menstruation, which is the shedding of the uterine lining when pregnancy does not occur. This process starts when hormone levels, particularly estrogen and progesterone, drop after ovulation if no fertilization has taken place. Menstruation typically lasts for a few days, and once it concludes, a new cycle begins with the follicular phase, during which the body prepares for the possibility of ovulation and potential fertilization.

In contrast, events like ovulation, fertilization, and implantation occur after menstruation has already started. Ovulation is the release of an egg from the ovary, which happens roughly midway through the cycle and signifies the peak time for potential fertilization. Fertilization is the union of sperm and egg, which occurs after ovulation, while implantation refers to the attachment of the fertilized egg to the uterine lining; both processes occur only if ovulation has happened and are part of the cycle but do not signify its start. Hence, menstruation distinctly denotes the commencement of a new cycle.