Placental Drug transfer ...The safety features

Transfer of drugs from mother to fetus takes place at the level of the placenta mainly by diffusion.

Thus, keeping maternal blood levels of drugs as low as possible is a major strategy for decreasing the amount of drug that reaches the fetus.

In addition, since 75% of the blood in the umbilical vein travels to the liver, a large portion of drug can be metabolized before reaching vital fetal organs.

Drug in the unbilical vein that travels via the ductus venosus to access the inferior vena cava will be diluted with blood from the lower extremities (which has no drug) and this further reduces concentration of drugs inthe fetal blood.

Two things work against this "safety feature":

(1) fetal acidosis during times of distress causes increased perfusion of the heart and brain and thus increases delivery of drug to these important organs.

(2) Fetal pH is lower than maternal pH and results in basic drugs (such as local anesthestics) becoming more ionized when they reach fetal circulation. This effectively traps them on the fetal side of the circulation since ionized molecules cannot easily cross the placenta. This also maintains a gradient for diffusion.

This is known as "ion trapping" and can be quite a significant effect especially during times of fetal distress (pH gets even lower).

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