Preterm birth affects nearly 1 in 10 pregnancies and is the leading cause of childhood death. Excessive maternal intrauterine inflammation is a major contributor to preterm birth. Overt inflammation in pregnancy is normally suppressed by maternal T regulatory (Treg) cells, with those exhibiting stable commitment to their cell lineage conferring optimal protection. A capability of some Treg cells to lose expression of the master transcription factor Foxp3, leading to instability in their commitment to the Treg cell lineage, has been observed in some tissue settings. While Treg cell deficiencies have been identified in women who deliver preterm, whether Treg cells can lose their lineage identity and adopt alternative, potentially inflammatory, phenotypes in preterm birth is unknown. In this study, we investigated the lineage stability of Treg cells in vivo in mice using a Foxp3 expression fate-mapping system and models of preterm birth induced by inflammatory challenge with lipopolysaccharide (LPS) or interleukin-1β (IL-1β) in late gestation. In non-pregnant mice and across normal gestation, ex-Foxp3-expressing (exTreg) cells were present in uterus-draining lymph nodes. Whereas Foxp3+ Treg cells increased in late gestation, when a larger proportion expressed the pro-inflammatory cytokines IFNγ and IL-17A, Foxp3- exTreg cell abundance and cytokine expression remained consistent. Bulk RNAseq of sorted Treg and exTreg cells revealed that exTreg cells are transcriptionally distinct from Treg cells, with substantial loss of the Treg cell lineage program characterised by reversal in expression of canonical Treg cell genes and pathways. Late gestation LPS- or IL-1β-induced preterm birth did not increase Treg cell conversion to exTreg cells. Therefore, Treg cells exhibit a high level of lineage stability in pregnancy in mice, with no increased conversion rate to exTreg cells throughout gestation even after inflammatory challenge. Despite this, the biological significance of the presence of exTreg cells in gestational tissues remains to be defined.