Follicle size varies during different periods of a woman’s menstrual cycle. In a normal woman during her reproductive years, only a few follicles develop and mature in each cycle, of which only one follicle undergoes ovulation, while the rest of the equally mature follicles do not ovulate and degenerate. Ultrasound is currently the most accurate method for follicle monitoring. On the 3rd-5th day of the menstrual cycle, ultrasound can observe small follicles in the ovaries, which grow gradually, on average by 1.5-2 mm per day, reaching their maximum on the 14th day before the next menstrual period, the day of ovulation. During the first half of the menstrual period, several follicles usually develop one after another and grow gradually. The largest and most mature follicle is called the dominant follicle, which is within 10-18mm. Some people need to wait until the follicles reach about 23mm before they are expelled, so there is some individual variability in the early and late ovulation. The size of the follicle can be monitored clinically to guide the timing of intercourse. The normal range is 1.8-2.3 cm when the follicle is mature. If the follicle is immature or mature but not expelled, conception is not possible.