A normal follicle should be between 18-23 millimeters on the 14th day of the menstrual period, or day 14 of the menstrual cycle. For women with regular menstrual cycles, the dominant follicle usually increases in size to about 18 millimeters on days 11-13 of the menstrual cycle, forming a preovulatory follicle. The preovulatory follicle is the final stage of follicular development and is a mature follicle, also called a Grave’s follicle. This is when the follicular fluid increases dramatically, the follicular lumen enlarges, and the follicle size increases significantly. By the 14th day of menstruation, the follicle has grown to 18-23 millimeters. If the menstrual cycle is long, the follicles may be relatively small on day 14 of menstruation and may not reach 18 millimeters. If the menstrual cycle is irregular, there may not be a dominant follicle on the 14th day of menstruation, as it is possible that the follicle may be underdeveloped.