Placental Transfer

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Most medications cross the placenta to the foetus. During gestation, the surface area of the placenta increases, while the placental thickness decreases to 1/5 at term. Both processes tend to favour the transfer of chemicals to the fetus.

Mechanism of placental transfer

Drugs, nutrients and other substances cross the placenta by …

• Simple diffusion e.g. most drugs
• Facilitated diffusion e.g. glucose
• Active transfer e.g. vitamins, amino acids
• Pinocytosis e.g. immune antibodies
• Breaks between cells e.g. erythrocytes

The last two are of no practical importance in drugs transfer

Factors influencing rate of transfer
• Molecular weight
• Lipid solubility
• Uterine and umbilical blood flow(major factor)
• Maternal diseases e.g. hypertension, diabetes