If the follicle diameter is less than 18mm, it means that the follicle is small or no mature egg is discharged, which seriously affects conception, and even if conception occurs, it increases the chance of fetal malformation or miscarriage. If the follicle diameter is greater than 25mm, it means that the follicle is large and is prone to follicular luteinization, which can affect normal conception. After menstruation, follicles begin to develop on day 5 or 6 of the menstrual cycle. If the body is producing enough estrogen and progesterone, follicles begin to grow at a rate of 2mm per day. On the 16th day of the menstrual cycle, 14 days before the next menstrual period, the follicles will develop to about 20mm, and then there will be mature and high quality eggs discharged.