Following fertilization, there is an initial period of rapid cell division that leads to the formation of a multicellular structure known as the blastula, or blastocyst. Within this structure, sialic acids play a key role in influencing cellular processes such as signaling, cell-to-cell contact, and adhesion. In species that develop internally, the blastocyst undergoes implantation and placentation, which depend on maternal immunomodulation facilitated by sialylated proteins and enzymes involved in the biosynthesis of sialic acids. Although research has shown that the elimination of certain initial enzymes in the sialic acid synthetic pathway can lead to reduced embryonic viability, the precise role of these enzymes remains to be further investigated, particularly in the blastula of externally developing species, which have received limited attention. Recently developed blastoid models present promising prospects for future research in this field.