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Normally a follicle gradually enlarges and approximately 14th day of the menstrual cycle, it ruptures releasing the ovum which has to get fertilized with the sperm in the Fallopian tube
, leading to pregnancy
Usually follicle ruptures when it attains the size of 18 to 25 mms. In your case it appears that the follicle failed to mature. There are medicines available for stimulating the follicle to mature, but these medicines have to be taken strictly under medical supervision.