Where does “placenta” come from?

placenta (Serbo-Croatian) comes from Ecclesiastical Latin placenta, from Latin placenta, from Ancient Greek πλακόεντα, from Ancient Greek πλακοῦς, from Ancient Greek πλακόεις, from Ancient Greek πλάξ, from Proto-Hellenic pləks, from Proto-Indo-European pleh₂- — flat.

placenta (Serbo-Croatian): placenta