The follicular phase is the period of time during which follicles are developing in the ovaries and refers to the period between the start of menstruation and just before ovulation. A woman’s menstrual cycle is generally divided into three periods: the menstrual phase, the follicular phase, and the luteal phase. The follicular phase is the period from the beginning of follicular growth to maturity, usually 14 days. The length of the follicular phase is variable, and its duration determines the length of a woman’s menstrual cycle. During the follicular phase, the follicle is stimulated by gonadotropins and gradually develops into a mature follicle, which can be up to 18 to 23 mm in diameter, and at the same time, under the action of oestrogen, the endometrium proliferates and develops, the glands increase in number and become longer, so as to prepare for the woman’s pregnancy. In addition, everyone’s menstrual cycle is different, and the timing of the menstrual phase, follicular phase, and luteal phase varies.