The carcinoembryonic antigen (CEA) family consists of a large group of evolutionarily divergent glycoproteins. The secreted pregnancy-specific glycoproteins constitute a subgroup within the CEA family. They are predominantly expressed in trophoblast cells throughout placental development and are essential for a positive outcome of pregnancy, possibly by protecting the semiallotypic fetus from the maternal immune system. The murine CEA gene family member CEA cell adhesion molecule 9 (Ceacam9) also exhibits a trophoblast-specific expression pattern. However, its mRNA is found only in certain populations of trophoblast giant cells during early stages of placental development. It is exceptionally well conserved in the rat (over 90% identity on the amino acid level) but is absent from humans. To determine its role during murine development, Ceacam9 was inactivated by homologous recombination. Ceacam9^/ mice on both BALB/c and 129/Sv backgrounds developed indistinguishably from heterozygous or wild-type littermates with respect to sex ratio, weight gain, and fertility. Furthermore, the placental morphology and the expression pattern of trophoblast marker genes in the placentae of Ceacam9^/ females exhibited no differences. Both backcross analyses and transfer of BALB/c Ceacam9^/ blastocysts into pseudopregnant C57BL/6 foster mothers indicated that Ceacam9 is not needed for the protection of the embryo in a semiallogeneic or allogeneic situation. Taken together, Ceacam9 is dispensable for murine placental and embryonic development despite being highly conserved within rodents.