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The placental barrier consists of a four-layer, semi-permeable membrane separating maternal and fetal circulation. When we say something crosses the placenta, we mean it passes through this membrane. Inside a placenta are caPillary-filled branches soaked by spumes of mother's blood. The placental barrier does an admirable job of keeping out bacteria, which are usually too large to pass into the placental branches. Special immune agents swiftly dispatch those that do manage to sneak through.

However, in recent years scientists have had to revise their perception of an impenetrable placental fortress. When it comes to toxic chemicals, the placenta is not really a barrier at all. Chemical substances carried in the mother's circulation are sorted by the placenta that are primarily based on molecular weight, electrical charge and fat solubility. In other words, small, neutrally charged molecules that readily dissolve in fat are afforded free passage regardless of their capacity for harm. In a sense it is a total free ride for all for toxins!