Recurrent pregnancy loss (RPL) is a common condition of largely undetermined pathogenesis. There is prior evidence of association with immune dysregulation linked to elevated NK cell levels and cytotoxicity. However, experimental findings remain contentious, hindering clinical adoption of immunological testing. Given their importance in healthy pregnancy, this study set out to determine the clinical utility of NK cell assays in 100 non-pregnant women with RPL and 80 healthy control women and establish an exploratory mass cytometry panel for in-depth NK phenotyping. As previously described, peripheral NK cell elevation was observed with RPL. The augmented NK cell cytotoxicity, often referenced, was undetectable, although enhanced degranulation was observed. Reduced cytolytic molecule secretion by PBMCs was seen in RPL, possibly counterbalancing the increased NK cell degranulation. Mass cytometry was employed for the detailed investigation of NK cell phenotype, focused on inhibitory and activating receptor expression. Augmented prevalence of CD57