Dynamics of maternal-fetal nutrient transfer.

Several mechanisms account for transfer of substances across the placenta, including passive diffusion, facilitated diffusion, active transport, solute drag, and pinocytosis. The rates at which these processes occur and the rates of equilibration of various substances between maternal and fetal circulation vary widely. In general, gases and some ions equilibrate rapidly while large molecules such as proteins exchange slowly. Some nutrients such as amino acids, calcium, iron, and possibly some vitamins are transported against a concentration gradient. This report compares equilibration rates for various nutrients and discusses the use of mathematical modeling to quantitate certain aspects of the processes, and to gain a better understanding of factors determining placental exchange.