How do most medications pass through the placenta?

Most medications cross the placenta primarily through passive diffusion. This process relies on the concentration gradient between the maternal and fetal blood. When the concentration of a drug is higher in the maternal blood than in the fetal blood, the drug moves across the placental membrane into the fetal circulation. This phenomenon occurs because the placenta is formed from layers of tissue that allow certain substances to pass while blocking others, and it does not require energy input, which is a defining characteristic of passive diffusion.

Factors affecting this transfer include the molecular weight of the medication, lipid solubility, and the degree of ionization. Generally, small, non-polar, and lipid-soluble drugs are more likely to pass through the placenta easily, which is why many medications given during pregnancy need careful consideration regarding their safety and potential effects on fetal development. While other transport mechanisms like active transport and facilitated transport exist and may play a role in moving certain substances, the majority of medications typically rely on passive diffusion under normal circumstances.