The second day of menstruation, the largest follicle 8mm is normal. Follicle development is affected by the ovarian endocrine function, there are individual differences, in the second day of menstruation, the largest follicle has 8mm with the usual short menstrual cycle, ovulation in advance there is a certain relationship, which will not affect the normal development of the follicle and ovulation, the usual menstrual cycle is longer ovulation is also relatively delayed. When the follicle develops to 18×20mm, it indicates that the follicle has matured, and the mature follicle has the possibility of being discharged at any time under the action of hormones, if there is a requirement for fertility monitoring to the mature follicle can be arranged for coitus to improve the chances of getting pregnant; for the women who do not have the requirement for pregnancy need to do a good job of contraceptive measures, so as to avoid unwanted pregnancy and abortion to cause bodily harm. If there are follicular development abnormalities, you need to seek timely medical attention.