Pregnant women are the most "untouchable" group of people in relation to pharmacological research due to ethical and legal aspects, as well as concerns for the health and integrity of the fetus. And that is why pregnant women practically do not participate in clinical, pharmacodynamic, or pharmacokinetic testing. The mechanisms of teratogenesis are unpredictable, and in this case mutations can occur regardless of the duration of pregnancy and at any level. In women during pregnancy, the activity of liver enzyme systems involved in drug metabolism changes completely, which affects their clearance. This should be taken into account when selecting drugs and dosages for the treatment of various diseases. Our study showed that during pregnancy, a significant decrease in the intrinsic hepatic clearance of the CYP1A2 substrate is enhanced by a decrease in the binding of theophylline to plasma proteins and an increase in the glomerular filtration rate.