Vaginal calculi have been described from the common (Delphinus delphis), Pacific white-sided (Lagenorhynchus obliquidens) and spotted (Stenella attenuata) dolphins. We describe additional calculi found in six sexually mature D. delphis from southern California. Three calculi were large (ca. 7×5 cm), exhibited concentric layer crystallization, and were unique from previously published descriptions. One calculus described previously and one in our sample appeared to be a fetal skeleton and skull respectively. Using CAT scans of a first trimester northern right whale dolphin (Lissodelphis borealis) and of a near term Delphinus delphis, we discuss the potential origin and development of vaginal calculi through analysis of ossification in embryonic delphinids. We hypothesize that the calculi represented spontaneous incomplete abortion with retention of part or all of the fetus in the distal reproductive tract. The form of the calculus relates to the degree of skeletal development at the time of fetal death. Calculi from a pregnant dolphin provided one measure of residence time.