Energy and substrate requirements of the placenta and fetus.

The exchange of nutrients between placenta and fetus involves three major mechanisms: (1) direct placental transfer of nutrients from the maternal to the fetal plasma, (2) placental metabolism and consumption of nutrients, (3) placental metabolism of nutrient substrates to alternate substrate forms. Additionally, placental size, architecture, developmental and pathological processes, and interaction with the fetus co-operate with transport and metabolic mechanisms to affect qualitsltively and quantitatively placentalfetal nutrient exchange. Most of the findings and concepts that are reviewed reflect animal experimentation, given the paucity of in vivo physiological information from human studies for obvious technical and ethical reasons (Hay, 1989).