Occurrence and type (resorption, abortion, stillbirth, mummification) of pregnancy loss in the dog and cat depend on the cause of the loss and the stage of gestation at which it occurs. General categories of known causes of embryonic or fetal death in the dog and cat include chromosome and developmental abnormalities of the fetus, infectious agents, maternal endocrine abnormalities, trauma, exogenous drugs, uterine torsion, and dystocia. Diagnosis of fetal loss and of cause of fetal loss relies on the history, physical examination, abdominal radiography and ultrasonography, serology, hormone assay, and on culture, pathology, and karyotype of fetal/placental tissue. Aggressive diagnostics at the time of fetal loss are essential to preventive strategies at subsequent pregnancies.