Figure 4. In human females, all primordial follicles are formed in the fetus between 6 and 9 months’ gestation. During this period, there occurs a marked loss of oocytes due to apoptosis. The number of primordial follicles decreases progressively as a consequence of recruitment, until very few if any are present after the menopause at ~50 years of age. (Baker TG: Radiosensitivity of mammalian oocytes with particular reference to the human female. Am J Obstet Gynecol 110:746, 1971. Reproduced with permission from Mosby, Inc.)