Maternal interleukin (IL) 10 deficiency elevates susceptibility to fetal loss induced by the model Toll-like receptor agonist lipopolysaccharide, but the mechanisms are not well elucidated. Here, we show that Il10 null mutant (Il10^−/−) mice exhibit altered local T cell responses in pregnancy, exhibiting pronounced hyperplasia in para-aortic lymph nodes draining the uterus with >6-fold increased CD4 and CD8 T cells compared with wild-type controls. Among these CD4 cells, Foxp3 T regulatory (Treg) cells were substantially enriched, with 11-fold higher numbers at Day 9.5 postcoitum. Lymph node hypertrophy in Il10^−/− mice was associated with more activated phenotypes in dendritic cells and macrophages, with elevated expression of MHCII, scavenger receptor, and CD80. Affymetrix microarray revealed an altered transcriptional profile in Treg cells from pregnant Il10^−/− mice, with elevated expression of Ctse (cathepsin E), Il1r1, Il12rb2, and Ifng. In vitro, Il10^−/− Treg cells showed reduced steady-state Foxp3 expression, and polyclonal stimulation caused greater loss of Foxp3 and reduced capacity to suppress IL17 in CD4 Foxp3⁻ T cells. We conclude that despite a substantially expanded Treg cell pool, the diminished stability of Treg cells, increased numbers of effector T cells, and altered phenotypes in dendritic cells and macrophages in pregnancy all potentially confer vulnerability to inflammation-induced fetal loss in Il10^−/− mice. These findings suggest that IL10 has a pivotal role in facilitating robust immune protection of the fetus from inflammatory challenge and that IL10 deficiency could contribute to human gestational disorders in which altered T cell responses are implicated.