Categorization of near-term mouse fetuses according to their viability as a tool for developmental toxicity screening

In 1,015 CD-1 near-term mouse fetuses, removed by cesarean section after cervical dislocation of dams, skin color, motility, crying, and breathing were studied. Three categories of fetuses were considered: living and surviving (92.5%); living but non-surviving (6.9%), and stillborn (0.6%). Particular attention was paid to the following: 24 fetuses, still apneic after routine manipulation, started breathing by specific maneuvers; 5 of these 24 fetuses were apparently dead when apnea recovery was successfully tried, and the living but non-surviving category must be considered in developmental toxicity screening as prenatal injuries may be manifested by an increase in the incidence of the fetuses belonging to this category.