Implantation and early placental development require the female immune system to tolerate the genetically disparate embryo. Maternal-fetal tolerance is primarily mediated by CD4⁺Foxp3⁺ regulatory T (Treg) cells. Insufficient numbers or suppressive function of Treg cells at embryo implantation causes infertility in mice and is implicated as a cause of reproductive disorders in women. The factors controlling the strength, quality and stability of the Treg cell response are not well defined. We recently demonstrated the hormone progesterone (P4) is a determinant of Treg cell abundance and phenotype. To further investigate the effects of P4 on Treg cells during pregnancy, mated female mice were administered various doses of the P4 antagonist, RU486, in the peri-implantation period. At mid-gestation, we observed decreased proportions of pregnant females with increasing RU486 treatment, along with decreased proportions of total Treg cells and increased proportions of IFNγ-producing Treg cells in the uterus-draining para-aortic lymph nodes. Pregnancy outcomes and susceptibility to late-gestation inflammation-induced fetal loss were measured in a second cohort of females treated with low-dose RU486, followed by lipopolysaccharide (LPS) or control in late-gestation. A substantial reduction in pregnancy rate was observed in RU486-treated females in late-gestation. Females treated with RU486 and LPS exhibited elevated fetal loss, and reduced fetal weight amongst viable fetuses. This work demonstrates P4 perturbation at implantation impairs Treg cell tolerance and subsequently compromises fetal development in later gestation, particularly if exacerbated by inflammatory challenge. We are now exploring the extent to which Treg cells account for the effects of reduced P4 signalling at implantation on pregnancy success. These findings are relevant to understanding the protective role of Treg cells in gestational disorders in women.