OBJECTIVE: To assess directly the dynamics of the formation of the ovarian reserve in the normal human ovary by evaluating the total number of follicles in developing ovaries when folliculogenesis occurs. DESIGN: Histomorphometry-based follicle counts in complete serial tissue sections. SETTING: Functional Anatomy Research Center, University of Milano. PATIENT(S): Thirteen fetuses, neonates, and one 8-month-old infant. INTERVENTION(S): Fifteen ovaries were completely cut, obtaining serial sections. Ovarian volume, volume fractions, density and total number of follicles per ovary were calculated using quantitative morphometric methods. MAIN OUTCOME MEASURE(S): Age-related dynamics of the establishment of ovarian reserve in human developing ovary at the end of the organogenesis. RESULT(S): The ovarian reserve (100,000 follicles at 15 weeks of postconceptional age) increased progressively to 680,000 follicles at 34 weeks. At 8 months of postnatal age the pool was still about 680,000 primordial follicles. CONCLUSION(S): The consistence of the primordial follicle pool during organogenesis shows an exponential increase until month 8 of prenatal life and it is subsequently maintained without modifications at least until month 8 of postnatal life.