BACKGROUND: Although extremely common, and with a broad range of associated negative outcomes for both parents and infants, the full impact and cost of perinatal mood and anxiety disorders (PMADs) is under-researched. Rural states, such as Montana, in particular experience gaps in access to mental healthcare and high rates of PMADs, emphasizing a need for an economic analysis of untreated PMADs. To address this gap, a cost analysis of the impact of untreated PMADs in Montana was conducted using 2021 birth data. METHODS: A cost-of-illness analysis was conducted to estimate the incremental costs of untreated PMADs in Montana. Secondary data sources and peer-reviewed literature were utilized to provide baseline estimates of the prevalence of maternal and birth outcomes in Montana and to estimate the impact of PMADs on these outcomes from the perinatal period through the first year postpartum. RESULTS: Using 15% as the PMAD prevalence rate in Montana, the total estimated incremental cost of untreated perinatal mood and anxiety disorders for births in 2021 is about 1.7 million. This is about 2,897 per impacted mother-child pair. DISCUSSION: This study provides needed insight into the economic impact of untreated PMADs and highlights the broad range of negative impacts PMADs have on the well-being of parents and infants. This study highlights the importance of providing effective treatment to people experiencing PMADs to improve the health of both parents and their infants and has important policy implications for increased funding and workforce development for the detection and treatment of PMADs.