In the human species, follicle growth is a very long process. The ovulatory follicle originates from a cohort of pre-antral follicles that have differentiated their theca interna 85 days earlier under the control of the high peri-ovulatory steroid and gonadotrophin levels. During growth the number of follicles decreases by atresia under the influence of factors relating to the size of the follicles. Their growth rate is regulated by intra- and extra-ovarian hormonal factors. At the end of the luteal phase, 15-20 days before ovulation, the next ovulatory follicle is recruited among a follicle population of 2-5 mm in diameter. At the beginning of the menstrual cycle it can be distinguished only by its size; a few days later it starts its maturation and exhibits its dominance on the other healthy follicles present in the two ovaries.