Which phase initiates the follicular phase of the menstrual cycle?

The follicular phase of the menstrual cycle is defined as the period that begins with the first day of menstruation and lasts until ovulation. This phase is characterized by the development of follicles in the ovaries, driven by the increase in follicle-stimulating hormone (FSH). The onset of menstruation marks the beginning of a new cycle, signaling the start of the follicular phase, as the body prepares for the possibility of ovulation and subsequent fertilization.

The first day of the period is significant because it not only indicates a decrease in hormone levels that leads to the shedding of the uterine lining but also triggers hormonal changes that promote follicle development. Hence, selecting the first day of the period is accurate, as it clearly demarcates the start of the menstrual cycle and the initiation of the follicular phase.