During pregnancy, the maternal immune environment has the capacity to influence fetal development and program altered phenotype of offspring. The pre-implantation period has emerged as the most vulnerable window in pregnancy for susceptibility to fetal programming imposed through maternal physiological and epigenetic perturbations. This results in altered embryonic gene expression, a transformed developmental program and ultimately consequences for the health of offspring. The underlying mechanisms remain to be fully elucidated. Progesterone (P4) signalling in the luteal phase and early pregnancy is a key driver of the maternal immune adaptation to pregnancy. We previously showed that luteal phase P4 signalling defect in C57Bl/6 female mice, achieved through administration of low-dose P4 antagonist RU486 on gestational days (GD) 1.5 and 3.5, compromises maternal immune adaptation causing significantly fewer uterine Foxp3+ T regulatory (Treg) cells and adverse fetal outcomes. In this study we aimed to investigate the impact of reduced peri-implantation P4 signalling-mediated immune disruption on the immune profiles of adult 16-week-old offspring. Flow cytometry was performed to assess T cell profiles in spleen, thymus and mesenteric lymph nodes from offspring of RU486-treated or carrier-treated dams. Strikingly, offspring exhibited altered T cell profiles with sex-specific differences, such that female offspring demonstrated prominent alterations in immune parameters. Most notable was a significant reduction in the proportion (P=0.0361) and total number (P=0.0343) of CD4+CD25+Foxp3+ Treg cells in the thymus of female offspring from RU486-treated mothers. Numbers of T conventional cells (CD4+Foxp3-) were unchanged however, indicating a specific loss of Treg cell generation. These novel results demonstrate that impaired P4 signalling during the peri-conception phase is a critical determinant of immune system programming evident in adult female offspring. A less tolerogenic immune phenotype increases susceptibility to a range of inflammatory conditions and so represents a new mechanism by which peri-conception fetal programming is mediated.