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The role of anergy, an acquired state of T cell functional unresponsiveness, in natural peripheral tolerance remains unclear. In this study, we found that anergy was selectively induced in fetal antigen–specific maternal CD4+T cells during pregnancy. A naturally occurring subpopulation of anergic polyclonal CD4+T cells, enriched for self antigen–specific T cell antigen receptors, was also present in healthy hosts. Neuropilin-1 expression in anergic conventional CD4+T cells was associated with hypomethylation of genes related to thymic regulatory T cells (Tregcells), and this correlated with their ability to differentiate into Foxp3+Tregcells that suppressed immunopathology. Thus, our data suggest that not only is anergy induction important in preventing autoimmunity but also it generates the precursors for peripheral Tregcell differentiation.