During a triinstitutional study to test whether individually ventilated caging systems impaired welfare and reproduction relative to static housing systems, varying numbers (2 to 7) of discoid-shaped, fleshy structures were found in utero of 17 postpartum female mice on study. Further investigation revealed these structures to be retained fetal membranes (RFM). A point prevalence of 24.3% was calculated based on a total population of 70 postpartum female mice on study. This finding was preceded by 3 typical clinical presentations, which are described here. We designed a case-control matched cross-sectional epidemiologic study to identify associated risk factors and antemortem indicators of RFM. Housing on the bottom shelves and attachment to the rack systems were factors associated with a diagnosis of the condition. In addition, neutrophilia, monocytosis, lymphopenia, and decreasing hematocrit values were associated with the diagnosis of RFM. These results confirmed that a CBC can be a useful antemortem screening test for the identification of affected mice. We conclude that RFM are likely an incidental finding although they may present concurrent with other pregnancy complications.