APA Dictionary of Psychology

the exchange of substances between mother and fetus, via the placenta, during gestation. The fetus is thereby supplied with nutrients and oxygen, and its waste products (e.g., carbon dioxide, urea) are eliminated. Substances of low molecular weight cross the placental barrier easily, but large molecules (e.g., proteins) do not; therefore, the fetus manufactures its own proteins from amino acids supplied by the mother. Some drugs (e.g., alcohol, opioids) as well as disease agents (e.g., the rubella virus) may cross the placental barrier and produce congenital defects.