Many stressors today are psychological rather than physical and are influenced by the brain's perception of the stressor. The peripartum period is a particularly volatile time that is susceptible to new and stronger stressors. This current study investigates the relationship between self-reported perceived stress levels and physiological cortisol reactivity levels in new mothers at the 12-week postpartum time point. In addition, it examines the relationship between the mother and infants' physiological cortisol reactivity levels at 12-weeks postpartum. This current study is part of a longitudinal study and assessed these two correlations for 181 mother-infant dyads from a low income Mexican American population. The self-reported stress levels were assessed using the 4-item Perceived Stress Scale (PSS) and the cortisol reactivity data was gathered using four salivary cortisol samples taken from both mother and infant surrounding 5 interaction tasks and analyzed using Area Under the Curve with respect to ground (AUCg). Unexpectedly, the results found no correlation between perceived and physiological stress levels in the mothers, with a Pearson correlation of 0.114 and a p-value of 0.129. However, there was a positive correlation between mother and infant cortisol reactivity, with a correlation of 0.632 and a p-value less than 0.0001. This early postpartum period plays a significant role in developing HPA axis regulation for infants and developing productive mother-infant interactions. The physiological and psychological risks of chronically elevated stress for both mothers and children were addressed in this study as well, with implications for means to address and mitigate potential cortisol dysregulation.