Which statement about placental transfer of opioids is most accurate?

Which statement about placental transfer of opioids is most accurate?

• A. Opioids do not cross the placenta.
• B. Placental transfer is limited to non-lipophilic opioids.
• C. The fetus rapidly metabolizes opioids to inactive forms.
• D. Opioids cross the placenta due to lipophilicity; once across, they become ionized in the fetal environment and accumulate, potentially causing ventilatory depression and dependence.

Answer: (D)

Opioids cross the placenta mainly because of their lipophilicity. They diffuse readily in their unionized form from maternal blood into fetal blood. In the fetus, the environment is relatively acidic, which protonates these weak-base opioids and traps them as ionized molecules in fetal circulation. This ion trapping leads to accumulation in the fetus, increasing the risk of neonatal ventilatory depression at birth and potential dependence/withdrawal after exposure. The other statements aren’t correct because opioids do cross the placenta; transfer isn’t limited to non-lipophilic opioids; and the fetus does not rapidly metabolize opioids to inactive forms—fetal metabolic capacity is limited, allowing the drug to persist.

Opioids cross the placenta mainly because of their lipophilicity. They diffuse readily in their unionized form from maternal blood into fetal blood. In the fetus, the environment is relatively acidic, which protonates these weak-base opioids and traps them as ionized molecules in fetal circulation. This ion trapping leads to accumulation in the fetus, increasing the risk of neonatal ventilatory depression at birth and potential dependence/withdrawal after exposure. The other statements aren’t correct because opioids do cross the placenta; transfer isn’t limited to non-lipophilic opioids; and the fetus does not rapidly metabolize opioids to inactive forms—fetal metabolic capacity is limited, allowing the drug to persist.