The syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nutrient circulation between the embryo and the mother.

It is the outer layer of the trophoblasts and actively invades the uterine wall, during implantation, rupturing maternal capillaries and thus establishing an interface between maternal blood and embryonic extracellular fluid, facilitating passive exchange of material between the mother and the embryo.

If they were to reach the fetal side of the placenta, many foreign proteins would be recognized, triggering an immune reaction.

By decreasing the translation of these gene products, the syncytiotrophoblast reduces the chances of an attack by the maternal immune system mediated by T-cells.

[4] Tony M. Plant, Anthony J. Zeleznik: "Knobil and Neill's Physiology of Reproduction: Two-Volume Set" p 1790