During gestation, CD4+/CD25+ regulatory T cells (TREG) play crucial role in mechanisms by which maternal immunological system tolerates semiallogeneic fetus. Paternal antigens as well as maternal hormons contribute to the pregnancy- associated increase of TREG population. TREG regulate immunological response by cell-cell contact and indirectly by cytokines. TREG population expands during human pregnancy, that may be detected both in peripheral blood and in decidua.