Under normal circumstances, the ovaries will have one follicle per month that matures and is expelled. Typically, a mature follicle may reach about 18-25mm in diameter. A follicle is considered to be mature when it reaches 18mm or more. However, there are individual differences and some follicles will release their eggs when they reach 15mm, while others will release their eggs when they reach 25mm, which can be monitored clinically by ultrasound. Normally there are multiple follicles developing at the same time at the beginning of each menstrual cycle, but usually only one follicle develops to maturity, called the dominant follicle, and the rest of the follicles are successively occluded. Ultrasound can detect small follicles in the ovary on day 3-5 of the menstrual cycle, and later the dominant follicle will gradually grow. By the time the follicles reach 18mm or more in diameter they are mature and ovulation may occur at any time. For women with regular menstrual cycle, ovulation monitoring is usually started 4-5 days before ovulation, for example, if the menstrual cycle is 28 days, monitoring will be started on the 9th day of menstruation and ultrasound will be done every other day to measure the size of the dominant follicle. Normally, follicles grow at a rate of 2mm per day. If the monitored follicle grows to 18mm or more, ovulation may occur at any time and the chances of conceiving with intercourse are higher at this time. If the ultrasound indicates that there are ≥12 or more small follicles on one or both ovaries, it is possible that polycystic ovary syndrome is present. Polycystic ovary syndrome is a pathology characterized by hyperandrogenemia, ovulation disorders and polycystic ovarian changes, often occurring in adolescent or fertile women, manifesting as prolonged menstrual cycles or amenorrhea, reduced menstrual flow, obesity, varying degrees of hirsutism, acne, etc. Because of the lack of ovulation, many women have infertility. If these manifestations are present, a hospital visit is recommended to check hormone levels, blood sugar and insulin for a full evaluation. Treatment of polycystic ovary syndrome is complex and is best done under the guidance of a physician. Women who have a need for contraception can take oral contraceptive pills or oral progestin during the second half of their menstrual period to regulate their periods. If you are planning to get pregnant, you can regulate your period with ovulation-promoting drugs. If you have a combination of hirsutism or acne, you can take Darex-35, which is also a contraceptive pill with a good androgen-lowering effect. In patients with combined obesity or insulin resistance, they can be treated with metformin. Patients with polycystic ovary syndrome should also make lifestyle adjustments, control their diet, exercise, and stop smoking and drinking.