Which of the following factors decreases placental transfer of substances?

Ionization is a significant factor that decreases the placental transfer of substances due to its impact on the solubility and permeability of compounds across the placental barrier. Many substances need to be in a non-ionized form to diffuse readily through cell membranes, including those of the placenta. When a substance is ionized, it becomes more water-soluble and less lipid-soluble, which hinders its ability to cross the lipid membranes that separate maternal and fetal circulation.

In general, non-ionized molecules can diffuse more effectively through the lipid bilayer of cell membranes, allowing for easier transfer across the placenta. Consequently, an increase in the ionization of a substance reduces its passive diffusion through the placenta, limiting its availability to the developing fetus.

On the other hand, factors such as increased lipid solubility and enhanced diffusion actually promote the transfer of substances, while high protein binding can limit the free concentration of a drug that is available to cross the placenta. Understanding these dynamics is crucial for comprehending how various drugs and molecules affect fetal exposure during pregnancy.