placental barrier

During pregnancy, the placenta is the customs checkpoint between mother and baby. The placental barrier is the layer of tissue separating maternal blood from fetal blood, across which nutrients, oxygen, waste — and unfortunately many drugs — must pass.

Unlike the tight blood–brain barrier, the placenta is a relatively permeable membrane: most lipid-soluble, low-molecular-weight, unionised drugs cross it by passive diffusion fairly easily, reaching the fetus. Large molecules (like heparin and most monoclonal antibodies in early pregnancy) and highly charged drugs cross poorly. The placenta also has efflux transporters such as P-glycoprotein and some drug-metabolising enzymes that offer the fetus partial protection.

Because the barrier is leaky, the guiding principle is caution: a drug given to the mother should be assumed to reach the fetus unless proven otherwise. The risk depends on the drug, the dose and the gestational age — the first trimester is most critical for malformations (teratogens), while later exposure may affect growth or cause withdrawal in the newborn.