It is normal for the follicle to be larger than 16-18mm in diameter on the 13th day of menstruation. For women with a 28-day menstrual cycle, a follicle larger than 16-18mm in diameter on day 13 is normal, but follicular development is greatly influenced by the menstrual cycle. For women with a 28-day menstrual cycle, small follicles can be found in the ovaries on days 3 to 5 of the cycle, and then gradually become larger with the number of days of menstruation, with the largest follicles being found on the 14th day of menstruation on average. The value of the change in follicle diameter on the 13th day of menstruation is significant for different women because the length of their menstrual cycle varies, and needs to be judged in relation to the specific number of days in the menstrual cycle. If you are planning to get pregnant, you can increase your chances of getting pregnant by choosing to have intercourse after the follicles have reached dominance.