Drugs administered to the mother may affect the fetus via placental transfer or may cause a maternal disorder that affects the fetus (eg, maternal drug-induced hypotension producing fetal hypoxia). All anesthetic and analgesic drugs cross the placenta to some degree. Flow-dependent passive diffusion is the usual mechanism.
Most anesthetic and analgesic drugs have a high degree of lipid solubility, low molecular weight (<500), and variable protein-binding and ionization capabilities. These characteristics lead to rapid placental transfer. Local anesthetics and narcotics (lipid-soluble, unionized) cross the placenta easily; neuromuscular blocking agents (highly ionized) do not.