Placental Barrier

The fetus of a pregnant woman receives biologically important substances, including nutrients, and excretes metabolic waste products through the placenta. Substances can move from maternal blood to fetal blood and vice versa by passing cell membranes. This places restrictions on the distribution of some chemicals, but in general the permeability of this "barrier" is determined by lipid solubility of the drug. Placental permeability to drugs is lower than in the liver or kidney but approximately equal to muscle tissue. Therefore psychoactive drugs, both clinically used and recreational, readily cross the placenta, and many of them are known to affect fetal growth and development. It is also well recognized that drugs capable of producing physical dependence in their users, such as opiates, induce symptoms of drug withdrawal in infants born to addicted mothers. Effects of many psychoactive substances on the fetus have not been studied to a sufficient degree, and the decision to use a drug during pregnancy must be made after careful weighing the possible benefits to the mother and risks to the fetus.

Structural abnormalities can be induced by drugs during the critical periods of fetal development. The most exemplary case of such an effect, called teratogenesis, is the drug catastrophy associated with the use of thalidomide. Thalidomide was marketed and used as a tranquillizer in the early 1960s. It was discovered too late that this drug, when consumed during the fifth through seventh weeks of pregnancy, greatly enhances the risk for abnormal limb growth in the fetus. The thalidomide case was, in fact, pivotal in the implementation of major safety measures in drug development that are currently in effect.

Later in pregnancy, drugs cannot elicit major structural abnormalities but can still have a negative impact, for example, by inducing fetal hypoxia. Because psychoactive drugs readily cross the placenta, it is not appropriate to speak of a placental barrier when focusing on psychopharmacology.

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