Regulatory T cells (Tregs) are CD 4+ T lymphocytes with regulatory and reparative properties. Tregs are functionally and differentially controlled by Foxp3 transcription factor, and they are considered a key factor inducing feto-maternal tolerance. Disruption of this unique type of time-limited immune tolerance leads to pregnancy complications including miscarriages, intrauterine growth retardation of the fetus, and premature delivery. Tregs' production and functional properties are influenced by the cytokine environment, especially by TGF-β1, IL-2, and retinoic acid. These pleiotropic factors are involved in the differentiation of tolerogenic populations of decidual cells. In the present work, a detailed immunophenotyping of Tregs was performed, and a determination of serum concentrations of TGF-β1 during physiological pregnancy was realized. The examined subpopulations of Tregs have shown different signs or markers of their functional and maturity properties. In some subpopulations, apparent changes in Treg counts were observed during pregnancy. Moreover, typical development was observed for TGF-β1 serum levels. The main goal of the current work was an examination of differences in Treg subpopulation counts in women with premature deliveries compared to females with full term births....