Problem During pregnancy, the maternal immune system needs to adapt in order not to reject the semi-allogenic fetus. Method In this review, we describe and discuss the role of regulatory T (Treg) cells in fetal tolerance. Results Treg cells constitute a T helper lineage that is derived from thymus (natural Treg cells) or is induced in the periphery (induced Treg cells). Treg cells are enriched at the fetal-maternal interface, showing a suppressive phenotype. In contrast, Treg cells are not increased in the circulation of pregnant women, and the suppressive capacity is similar to that in nonpregnant women. However, aberrations in Treg frequencies and functions, both systemically and in the uterus, may be involved in the complications of pregnancy. Conclusion Treg cells seem to have distinguished roles locally versus systemically, based on their distribution and phenotype.