The size of follicles varies with a woman’s menstrual cycle, and the normal range for the size of mature follicles is 18 to 23 mm. At the beginning of the next menstrual cycle, the primary follicles start to develop. On the 7th day of the menstrual cycle, usually only one follicle develops into the dominant follicle, and the other follicles gradually degenerate and become atretic, and the dominant follicle grows to 18~23mm on the 11th~13th day of the menstrual cycle, when normal ovulation occurs, and the fertilization rate at this time is high. However, it is not absolute, some patients with follicles smaller than 18mm or larger than 23mm can also have ovulation. If there are a lot of small follicles in the ovary, no dominant follicle growth, or follicles more than 23mm or even larger and still do not ovulate is anomalous, the former may be polycystic ovary syndrome, the latter may be the follicle luteinization. For patients with no dominant follicle growth and long-term non-ovulation, it is important to go to a regular hospital in time for treatment under the guidance of a specialist.