Each month at the onset of the menstrual period, between 10 and 20 follicles will normally reach the antral stage, at which they are ready to grow into mature follicles under the influence of those hormones. Hundreds of other follicles that started down the developmental path at approximately the same time have regressed and have been reabsorbed by the body in the process known as atresia.
Once at the antral stage, the follicles require a certain level of FSH to survive and grow. In prepubertal girls and women taking birth control pills, without high enough levels of FSH to enable them to grow, the small antral follicles remain dominant. In a normally cycling woman, however, FSH levels begin to rise at the start of each menstrual cycle, and, under that influence, the antral follicles begin to grow. One of them will generally be slightly ahead of the others in its development, which gives it the advantage of being slightly more sensitive to FSH, which in turn causes it to grow faster than the other follicles and widen its lead.