What marks the first day of the menstrual cycle?

The first day of the menstrual cycle is marked by the beginning of menstrual flow. This day is significant because it represents the shedding of the uterine lining that occurs when pregnancy does not happen, leading to a new cycle. The menstrual cycle is typically divided into phases, including the follicular phase and the luteal phase. However, those phases commence only after the menstrual flow has started. The onset of ovulation also occurs later in the cycle, making it clear that the initiation of menstrual flow serves as the definitive marker for the first day of the cycle. Therefore, recognizing the start of menstrual flow as the first day is critical for tracking reproductive health and understanding the cycle's timing.