Abstract: It is necessary to develop the proper fetal -maternal immune tolerance for successfulpregnancy. Along with the development of pregnancy, progesterone, as a kind of immune regulatory factor, actson end-ometrial epithelial and stromal cells and spiral arteries, which provides the uterine micro-environmentfor pregnancy. Meanwhile, progesterone regulates the activities of dendritic cells, T cells and natural killer cellsin deciduas, and up -regulates human leukocyte antigen G gene expression. The effects of progesteronesynthetically induced the immune tolerance. Thus, progesterone is critical for the maintenance of pregnancy.