Metabolism in pregnancy
Many of the drugs that can reach and have negative effects on the fetus are metabolically eliminated. Unfortunately, the physiological changes that occur in pregnancy can lead to either increases, decreases or no-change in drug metabolism from pre-pregnant states. Therefore, it is recommended that each drug be reviewed for possible changes in metabolism.
Changes in enzymatic activity are thought to be related to pregnancy-associated hormonal changes. N-demethylation e.g. is inhibited by progesterone, but not by estrogen. The hepatic enzyme CYP3A4 has an increased activity during pregnancy and accelerates hydroxylation. Another hepatic enzyme, CYP2D6, has also alterations in activity during pregnancy and can increase the clearance of certain drugs.