In many cases the decisions on drug therapy during pregnancy have to be made without evidence-based information about the effectiveness and safety of the treatment. While few drugs are known with certainty to be harmful for fetal development, the evidence for evaluating harm to the fetus is insufficient for the majority of drugs. The differentiation of fetal organs takes place during the early weeks of pregnancy, whereby it is imperative that the mother's medication be revised already when planning a pregnancy. A drug should primarily be chosen, for which experience has accumulated about its use during pregnancy and is not suspected or known to be associated with adverse effects.