The maternal immune profile during pregnancy is implicated in programming fetal development to impact neonatal and long-term health, and the periconception period is particularly vulnerable. In this study we aimed to develop a model of maternal viral mimetic-induced immune activation during periconception to investigate the impact on pregnancy success, offspring development and immune parameters. Polyinosinic:polycytidylic acid (poly I:C) is a synthetic dsRNA, widely used to mimic viral infection. Female mice received 10 mg/kg poly I:C or control on gestational days (gd) 0.5 and 2.5. Flow cytometry analysis of the uterus, uterine-draining lymph nodes and spleen on gd3.5, identified a significant increase in pro-inflammatory T conventional cells and T regulatory cells, confirming maternal immune activation. This periconception inflammation resulted in a significant 18.5% decrease in fetal weight on gd17.5 and a significant 32.5% increase in placental:fetal weight ratios (p<0.0001), demonstrating fetal growth restriction and placental insufficiency. Postnatally, male and female offspring from poly I:C treated dams exhibited alterations in development, with elevated weight gain compared to offspring from control treated dams (p<0.05 & p<0.01 respectively) across a 12-week period. Flow cytometry was performed on 12-week-old offspring following lipopolysaccharide immune challenge. Strikingly, our preliminary data indicates that female offspring from poly I:C treated dams exhibit a less tolerogenic T cell profile with approximately a 10% reduction in the proportion of anti-inflammatory T regulatory cells in the spleen and mesenteric lymph nodes (p=0.0463 & p=0.0101 respectively). These results confirm the vulnerability of the periconception period and demonstrate that maternal inflammation around the time of conception 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 and autoimmune conditions and thus education on pre-conception health warrants further attention.