Human immunoglobulin G (IgG) Fc receptors are important in the materno-fetal relationship. Three classes of IgG Fc receptors are recognized which generate multiple isoforms, most of which are expressed in different cellular components of human placenta at different times during pregnancy. Although the distinct biological functions of Fc gamma R phenotypes expressed in human placenta are still unknown, recent data provide evidence for an important association between the Fc gamma R phenotype and transcytosis of IgG in the placenta. Selective transfer of maternal IgG across the placenta provides passive immunity to the fetus during the period when its own immune system is gaining protective potential. Furthermore, placenta-specific macrophages may contribute through Fc gamma R-mediated phagocytosis to the protection of the fetus from either infection or maternal immune attack against paternally inherited fetal antigens. Ontogeny and expression of various isoforms of Fc gamma R subtypes may be the key to the elucidation of the transport mechanism of maternal IgG to the fetus, in addition to the determination of the mechanisms of placental protection of the fetus against the maternal immune system.