Within the ovary, there is a vast oversupply of oocytes, where 99.9% of the ovarian reserve (OR) is destined for atresia. This seemingly wasteful system raises the question; why are there so many oocytes in the ovary? Until now, OR quantification has been limited to a single-end-of-life measurement and unable to address this long-standing question. Currently, primordial follicle counts require removal of ovary tissue but we have developed a method to quantify OR in live 25-day-old mice for the first time. Thus, we aimed to determine if the number of primordial follicles in the ovary at 25d influenced lifetime reproductive output. In Figla-CreTg/0-tdTomatofl/0b mice, which express fluorescent tdTomato in oocytes, the primordial follicles can be visualised through the surface of the ovary. This allowed us to surgically externalise and image the ovaries of 27 mice for OR quantification. Once the mice recovered from surgery, they were entered into a fertility trial until 13 months of age. OR was again quantified at this timepoint to examine rate of OR loss during reproductive decline. We monitored the pup production, inter-litter interval, and age at last litter across life to generate a comprehensive fertility dataset. Surprisingly, early-life OR was not associated with any of the fertility metrics. We are currently quantifying the end-of-life OR to be compared to the early-life OR. We will determine if the rank order of OR size in young females is preserved across life, as it has never been experimentally shown that early life OR determines when the ovarian reserve is depleted. Thus far, we have been able to show that a large OR early in life does not guarantee a prolonged reproductive lifespan, and for the first time, this technique has made it possible to quantify the OR at multiple ages in the same animal.