Depending on the development of the follicles, they can be divided into the following stages: the primordial follicle, the pre-sinus follicle, the sinus follicle and the pre-ovulatory follicle. The primordial follicle is the only form of oocyte reserve, and each of us has about 2 million primordial follicles at birth. Most of the primordial follicles gradually degenerate during childhood, and by puberty, there will be a total of about 200,000-400,000 primordial follicles left on both sides of the ovary, and only about 400-500 follicles will mature during a woman’s lifetime. The pre-sinus follicle is developed from the primordial follicle and consists of several developmental stages, including the primary follicle and the secondary follicle. The pre-sinus follicle needs to undergo nearly three menstrual cycles to develop into a sinus follicle. During the late luteal phase of the previous menstrual cycle and the early follicular phase of the current menstrual cycle, a group of sinus follicles (about 3-11 follicles) will enter into the growth trajectory together under the action of follicle-stimulating hormone (FSH) in the ovary, while the other follicles will be atretic, a process called recruitment. The growth of the recruited follicles is mainly dependent on gonadotropins, especially follicle-stimulating hormone (FSH), and the follicles can continue to grow only if the FSH level reaches or exceeds a certain threshold. Around day 5-7 of the menstrual cycle, the follicle with the lowest FSH threshold, i.e. the one that is most sensitive to FSH, will be prioritized to develop into the dominant follicle, while the other follicles will gradually atresia. There can be more than one follicle in a cycle, but usually only one follicle will eventually mature and ovulate, and this follicle is often referred to as the dominant follicle. For women with regular menstruation, the dominant follicle can usually be detected by ultrasound around the 8th day of menstruation, and the ultrasound can monitor its gradual growth to a diameter of about 17~24mm, when the follicle matures. After ovulation, the dominant follicle is visible on ultrasound and disappears, and the ovulated follicle forms the corpus luteum.