The number of female germ cells in humans undergoes three distinct rate changes, as diagrammed here and defined in the model. For the sake of clarity, the age coordinate is expanded artificially during the embryonic phase. The proliferative phase initiates at the time of germline-soma separation (ca. 9 days after fertilization; t = 0.0 year) and ends after 5 months of gestation (t = 0 – 0.42 year). The declining phases begin later in fetal life and continue into adulthood (t = 0.42 – 52 years) with an accelerated rate of oocyte depletion beginning at age 38 . The dotted line shown during the embryonic stage emphasizes that oogonial cell counts from this period are inaccessible to reliable determination.