Abstract: Objective To investigate the sources and mechanisms of regulatory T cells at the maternal-fetal interface. Methods Thymus, peripheral blood, spleen, lymph nodes and deciduas were harvested from different pregnancy periods （E0 for embryonic day 0, E4.5, E7.5, E10.5, E13.5, E16.5, E19.5）mice. The proportion of CD4⁺Foxp3⁺ Treg cells among CD4⁺ T cells in each sample was determined by flow cytometry. Real-time quantitative polymerase chain reaction （RT-PCR） and Western blot were used to detect the mRNA and protein expression level of chemokines （CCL2, CCL7, CCL12 and CCL20） in uterine and decidua of non-pregnant and E13.5 mice. Expression of CCR2 and CCR6 in Treg cells of peripheral blood of non-pregnant and E13.5 mice were tested by flow cytometry. Their chemotaxis to Treg cells during pregnancy was assessed by an in vitro chemotaxis assay in a transwell chamber. Results Compared to non-pregnant mice, the proportion of Treg cells in CD4⁺ T cells in thymus from syngeneic and allogeneic pregnant group was increased at E10.5 and E13.5, but there was no difference between them. The proportion of Treg cells in CD4⁺ T cells of allogeneic pregnant mice increased significantly and was higher than syngeneic pregnant group, while reached a peak at E10.5. Treg cells of both the two pregnant groups were higher than normal non-pregnant mice. The mRNA and protein expression level of CCL2, CCL12 and CCL20 in uterine and decidua of E13.5 pregnant mice were increased to non-pregnant mice, but the increase of CCL7 occurred in mRNA not protein. The expression of CCR2 and CCR6 in the surface of Treg cells in peripheral blood significantly enhanced in syngeneic and allogeneic pregnant group than non-pregnant one. CCL2, CCL7, CCL12, CCL20 exerted obvious chemotaxis on Treg cells during pregnancy. Conclusions Thymus maybe the main sources of Treg cells at the maternal-fetal interface and Treg cells arrived there by chemotaxis.