The aim of this paper was to obtain the precise answer about the effect of therapy by gestagens, that are intensively applied to prevent threatening spontaneous abortions. The frequency of micronuclei (MN) in samples from patients to whom appropriate therapy has been applied during the first 3 months of pregnancy was analyzed. The samples were from 29 phenotypically healthy pregnant women, average age of 26 ± 1 years; frequency of MN was determined before and after the end of therapy. Gestormone, Progesterone depo and Gravibinone have increased MN frequency significantly, with a probability of p<0.05, and Utrogestane has also increased MN frequency, but without statistical significance (p<0.1). ANOVA showed that the mentioned gestagens increase variability of MN frequency, with high statistical significance (F = 8.63; p<0.0048). Environmental factors (79%) influence the formation of MNs and their variability in great proportion.

Key words: gestagens, in vivo, lymphocytes, micronucleus, pregnancy, variability.