If the follicle diameter is close to 15mm, it is already close to the dominant follicle, basically the diameter of the follicle will be around 18mm when checked every other day. Since the average follicle growth rate is 1.5-2mm per day under normal circumstances, it is recommended to have intercourse when the follicle diameter reaches about 18mm, so that the chance of pregnancy will be higher. Because the sperm can survive in the woman’s body for up to 2-3 days, the follicles will basically ovulate every other day after they reach dominance and the sperm and egg will unite to form a fertilized egg. If you are not planning to get pregnant, it is better not to have intercourse at this time, if you want to have intercourse, you can choose condoms to give contraception, or take oral emergency contraceptive pills within 72 hours after intercourse. Therefore, it is very helpful to know the growth rate of the follicles in time to guide intercourse and improve the chance of pregnancy.