Explain why counting placental scars produces an overestimation of actual litter size?

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Counting placental scars within the uterus of a breeding female is a time-honored wildlife biology technique for estimating populations in mammalian species. In mammals that have litters originating from multiple eggs, each offspring has its own placenta; the location where the placenta is attached to the lining of the uterus leaves a permanent scar after the offspring is born. Theoretically, dissecting an animal and counting the scars in her uterus should tell you have many offspring she has delivered. This, however, does not tell you litter size unless you also know how many litters she had. If the last litter was not too long in the past, then the newest set of scars may look different, which does help with accuracy, but counts obtained this way may still result in overestimates because offspring that were aborted or reabsorbed will still leave scars.

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