The human ovarian reserve: the narrative and the science.
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BACKGROUND: The ovarian reserve is the quantity of non-growing primordial follicles (NGF) in paired ovaries. A low or high ovarian reserve at birth respectively may foreshadow early or late age at menopause. In textbooks, published papers, and internet modalities, a recurring theme is that for human females, on average, mid-gestation marks the peak supply of 7 million germ cells/NGF followed by large-scale depletion to about 1-2 million in paired ovaries at birth. OBJECTIVE AND RATIONALE: We test this narrative against the evidence for germ cell quantity and degeneration reported in peer-reviewed publications. Ovary sizes, volumes/shapes are particularly variable among same-age foetuses and at birth. Coupled with genomic mechanisms determining germ cell proliferation/differentiation/survival, substantial variations of germ cell numbers are recorded within and between individual studies. Across published papers, textbooks, and webpages, germ cell numbers in foetal-newborn ovaries range from thousands to 20 million. A massive 70-85% die-off among germ cells in foetal ovaries is reported during the second half of gestation. Although germ cell degeneration is a reality, we review also the evidence for the timing and extent of germ cell death in foetal/newborn human ovaries. SEARCH METHODS: We cite primary publications of human embryonic, foetal/newborn ovaries where data on germ cell numbers and death are reported, and similarly for studies of ovarian volume, shape, histology, and growth in foetal-early postnatal life. Searches to September 2025 used PubMed, Google Scholar, and DOIs/URLs from published papers, textbooks, and webpages, including keywords listed below. Textbook and webpage citations are a selection and not comprehensive, serving to illustrate the widespread narrative on human ovarian development. OUTCOMES: Germ cell number estimates (oogonia, oocytes, NGF) in human embryonic, foetal, and newborn ovaries (n = 139) in seven studies from 1953 until 2011 used three different quantitation methods: (i) volumetric/model-based with correction factors, (ii) volumetric/modified stereology, and (iii) fractionator/optical dissector. In a 1963 study, germ cells in paired foetal ovaries at 20 weeks (n = 2) reported 6.8 million in total with 20% atretic, and at birth (n = 2), 2 million in total with 50% atretic, leading to the narrative that the mid-gestation human female foetus has 7 million germ cells/NGF that are subsequently depleted to 1-2 million by birth. Independent investigations of germ cell quantitation/degeneration do not confirm these findings for total numbers nor the substantial fractions reported as degenerating. In adult women, ovarian volume is strongly correlated with the numbers of NGF but an equivalent correlation between germ cell supply and ovarian volume during foetal life up to birth has not been investigated. We conclude that the narrative whereby human foetal ovaries develop millions of germ cells followed by most degenerating up to birth has not been verified. Systematic analysis of total numbers and estimates of viable versus degenerating germ cells across gestation is needed. WIDER IMPLICATIONS: Does it matter how many germ cells or primordial follicles are in the ovaries at birth? Yes, not only with regard to scientific authenticity but also for clinical investigations of gynaecological and reproductive conditions. These include situations where follicle numbers and growth dynamics are of consideration, such as premature ovarian insufficiency, early menopause, chemotherapy for cancer treatments, polycystic ovary syndrome, assisted conception and/or IVF particularly for poor responders to gonadotrophins and predictions of age at the menopause. For fertility counselling and for women electing to delay pregnancy, it is suggested that clinicians and health professionals be aware that the age-related ovarian reserve in adults may not necessarily be in the expected range if the NGF reserve at birth was significantly lower than the narrative of 1-2 million at that time. REGISTRATION NUMBER: N/A.