Which statement best describes placental transfer of drugs?

• A. Ionized drugs cross more easily.
• B. Non-ionized, lipophilic drugs cross placenta more readily.
• C. Very high molecular weight ensures crossing.
• D. Strong binding to albumin ensures crossing.

Answer: (B)

Drugs cross the placenta mainly by passive diffusion, so the properties that promote diffusion across the placental membranes are key. Non-ionized, lipophilic (unionized and lipid‑soluble) molecules readily partition into the lipid components of the placental barrier and diffuse into fetal circulation. If a drug is ionized, it carries a charge and becomes more hydrophilic, making diffusion through the lipid membranes much less favorable. Very high molecular weight also hinders passage, and when a drug is bound to albumin, only the free fraction can diffuse, further limiting transfer. Because of these factors, non-ionized, lipophilic drugs cross placenta more readily than ionized, hydrophilic ones, making that description the best fit.