Objective:To analyze the effect of spleen on the Treg cells during pregnancy. Methods: The mononuclear cells were separated from the peripheral

spleen and uterus or placenta blood. Flow cytometry was employed to analyze the percent of Treg cells in total T cells in different stages of pregnancy. Immunohistochemical staining was used to make sure the distribution of Treg in spleen in different stages of pregnancy. Results: The results of immunohistochemical staining showed that compared with spleen Treg cells in normal unpregnant mice

spleen Treg cells on day 7 and 14 of pregnancy significantly increased. After splenectomy

peripheral blood and placenta Treg cells on day 7 of pregnancy markedly decreased as compared with the normal pregnancy（P<0.01）. And the cells on day 14of pregnancy were markedly recovered as compared with the normal pregnancy. Conclusion: Our study indicated that the spleen and its Treg cells might play important roles in transient tolerance during pregnancy.