Simple Summary: The anti-vaccination movement claims an alleged danger of the COVID-19 vaccine based on the presupposed similarity between syncytin, which plays a role in human placentation and the SARS-CoV-2 spike protein . We argue that because of very low sequence similarity between human syncytin-1 and the SARS-CoV-2 S protein, it is unlikely that any S protein-specific SARS-CoV-2 vaccine would generate an immune response which would affect fertility and pregnancy . However, further evaluation of potential impacts of COVID-19 vaccines on fertility, placentation, pregnancy and general health of mother and newborn is required . Abstract: Human placenta formation relies on the interaction between fused trophoblast cells of the embryo with uterine endometrium . The fusion between trophoblast cells, first into cytotrophoblast and then into syncytiotrophoblast, is facilitated by the fusogenic protein syncytin . Syncytin derives from an envelope glycoprotein (ENV) of retroviral origin . In exogenous retroviruses, the envelope glycoproteins coded by env genes allow fusion of the viral envelope with the host cell membrane and entry of the virus into a host cell . During mammalian evolution, the env genes have been repeatedly, and independently, captured by various mammalian species to facilitate the formation of the placenta . Such a shift in the function of a gene, or a trait, for a different purpose during evolution is called an exaptation (co-option). We discuss the structure and origin of the placenta, the fusogenic and non-fusogenic functions of syncytin, and the mechanism of cell fusion . We also comment on an alleged danger of the COVID-19 vaccine based on the presupposed similarity between syncytin and the SARS-CoV-2 spike protein.