The average size of a mature follicle on day 10 is approximately 16 to 25 mm. The process of ovulation begins during menstruation where three to five follicles are recruited, and just one is selected to be the dominant follicle.
During days 1 through 5 of a females menstrual cycle, selected or recruited follicles that are less than 5 to 10 mm in size regress and become atretic. During this follicle selection period, the goal is to help grow the strongest ones, so that one dominant one can be selected.
During days 5 through 7, a follicle that is of 10 mm in size becomes dominant. It will then grow 2 to 3 mm in size each day. By day 10, the follicle should be anywhere between 16 and 25 mm in size. Just prior to ovulation process, the follicle should be between 17 and 27 mm in size.
When ovulation begins, the follicle takes on a rapid growth spurt and begins protruding from within the ovarian cortex. The follicle attains a crenated border and explodes the ovum and antral fluid from the ovary.
The selection of the follicle does not guarantee that it will ovulate. There are a variety of factors that stop the follicle from producing ovum.