The fact that the cells of fetal tissues strongly divided, and drugs may adversely affect the process of cell division or break the link between them, which inevitably leads to violation of synthesis of tissues, organs, and in the early pregnancy, even to the maldevelopment of the limbs. Teratogenic effects drugs is not limited to physical underdevelopment, but may slow mental development. Therefore decided to allocate 5 most dangerous periods:

1. from the moment of conception to the 10 th day in this period, the toxic action drugs, usually leads to death of the fetus;

2. from 11 th to 28 th day (the period of organogenesis) in this period appears actually teratogenic drugs; particular type malformations due to pregnancy, ie a specific organ or functional systems formed during the toxic effects medications; in most cases after formation of an organ or functional system of medicine is not affects them actually teratogenicity;

3. from the beginning of 4 th week before nachala9 th; usually drugs drugs do not actually have teratogenic effects, but high risk delay the development of the fetus;

4. 9 th week before the end of pregnancy, at which time medical drugs usually do not cause delays in the development of organs and tissues of the fetus, but in result of their toxic effects on the fetus may be in breach of the functions of and systems in the postnatal period and implementation of various behavioral anomalies;

5. period immediately preceding the birth, or during childbirth; inappropriate use of drugs at this time could affect During the neonatal period. For example, the use of simpatolitika reserpine immediately before birth may lead to airway obstruction, anorexia and lethargy newborns.