If the menstrual cycle is regular, with an average cycle of 28-30 days once, the follicle should be between 18-20mm in diameter on the 15th day of menstruation. As the follicle grows, it will gradually move from the inside of the ovary to the surface of the ovary. When the follicle reaches a diameter of about 20mm, it has moved to the surface of the ovary and, due to the increase in internal pressure, ruptures and expels the egg. Therefore, on the 15th day of menstruation, the follicle has already reached the dominant follicle and may be expelled at any time. If you are planning to have a baby, it is recommended that you choose to have intercourse on the 15th or 16th day of your period when the follicles have reached dominance.