There is much evidence that cytokines play a very important role in the maintenance of pregnancy by modulating immune and endocrine systems. Placental tissue produces cytokines and hormones that are essential to the regulation of the feto-maternal unit. Decidual lymphocytes express cell surface markers for activation, such as CD69 and HLA-DR, and these cells secrete many cytokines. Recent studies suggested that in pregnant women, cytokines produced by Th2 cells predominate over those produced by Th1 cells, resulting in the maintenance of pregnancy. This review article focuses on the unique cytokine network at the feto-maternal interface in humans. Recently, we demonstrated that Th2 cells were dominant within the decidua in early pregnancy in humans. The Th2-derived cytokines, IL-4 and IL-6, induce the release of hCG from trophoblasts, and the hCG stimulate progesterone production from corpus luteum in pregnancy. Progesterone stimulates the secretion of Th2 and reduces the secretion of Th1 cytokines. Thus, Th2 type cytokines appear to contribute to the maintenance of pregnancy by controlling the immune and endocrine systems and promoting the function of the trophoblasts at the implantation site.