Prescription of drugs to a pregnant woman poses a challenge to physicians. Despite the scientific evidence of the harmful effects of certain drugs during pregnancy, a large number of pregnant women take relatively a lot of drugs. The thalidomide disaster was the most important event that debunked the myth of the placenta as a protective barrier and encouraged more detailed testing of drugs for use in pregnancy. ----- In pregnancy, changes occur in the cardiovascular, respiratory and gastrointestinal tract, in liver and kidney function, along with the changes in the activity of protein transport and enzyme systems. The most important factors affecting the diffusion of the drug through the placenta are: physical and chemical properties of the drug, the duration of a drug exposure, the characteristics of the distribution in fetal tissues, the stage of the development of the placenta and fetus, and the overall effect of the combination of drugs. In order to be teratogenic, a substance must cause a characteristic malformation, show the effect at a certain stage of gestation and have a dose-dependent effect. Adverse effects other than structural malformations include intrauterine growth restriction, miscarriage, stillbirth, and impaired cognitive development. Clinical trials of medications for use in pregnancy are based on case reports, epidemiological studies, registries of congenital malformations, pregnancy registries, population databases and other medical records. ----- Drugs with proven teratogenic or other harmful effects on fetus are contraindicated for the use in pregnancy. Examples of such drugs are ACE inhibitors, cytotoxic agents, diethylstilbestrol, radioactive iodine, retinoids, thalidomide, tetracyclines. The most common groups of drugs that are prescribed to pregnant women are medications for cardiovascular diseases, analgesics, anxiolytics, antibiotics and drugs for the treatment of thyroid disease. Perinatal complications such as nausea and vomiting and preterm birth also require the use of an appropriate medication. The most important task of a doctor is to assess whether the benefit of a drug for a mother outweighs the risk that a drug poses to the fetus. In the same way a medication must not be withheld if it is important for the health of a pregnant woman.