During pregnancy, the heavily glycosylated surfaces of the implanting blastocyst and maternal uterine epithelium interact in a highly controlled and specific manner. Examination of this interface in species that show interdigitation of embryonic and maternal surfaces (epitheliochorial placentation) shows that each has its own particular pattern of glycosylation or glycotype, and that closely related and/or interbreeding species e.g. horse and donkey or llama and guanaco, have very similar glycotypes. Implantation of interspecies hybrids is facilitated, when the blastocyst has an outer cell layer bearing glycans that are compatible with the maternal host. We refer to this mutual compatibility as a glycocode. The probability that hybrid embryo glycotypes differ from those normally associated with the host species may account for the high pregnancy failure rates seen in interspecies breeding. We suggest the maternal host selects between genotypically distinct embryos, and this selection depends partly on cell surface glycosylation. We infer that the glycocode plays a critical role in implantation, for if the survival of modified genotypes results in fitter offspring with altered placental glycosylation, selection pressure downstream may in turn act to drive adaptations in the maternal surface glycotype to produce a complementary glycocode, thus leading eventually to the creation of new species. We speculate that glycan microheterogeneity plays a specific role in this process.