Small follicles can be seen developing on the second day of menstruation, and normally the dominant follicle can be seen on the 8th-10th day of menstruation, so follicle monitoring is done for the first time from the 8th-10th day of the menstrual cycle. The follicle usually develops and matures in the middle of the menstrual period, i.e., on the 14th-15th day of menstruation to form ovulation, and the egg meets the sperm in the fallopian tube to form a fertilized egg. If the discharged egg does not have the opportunity to meet with sperm, the follicle is eliminated to form the corpus luteum, which atrophies to form the menstrual period and then menstruation occurs. In the late corpus luteum, small follicles begin to be recruited and begin to develop, along with changes in ovarian hormones, and the egg is prepared for the next menstrual cycle.