Background: Placental inflammation, defined as exposure to uncontrolled inflammation before birth, is prevalent in preterm birth (PTB) making up 10-15% of all births in the United States; it is a major cause of neonatal morbidity/mortality. Unfortunately, there are currently no available therapies targeting placental inflammation. Thus, there is great need to understand the cell-specific mechanism of prenatal inflammation at the placenta for the development of targeted therapeutics.  Methods: Retrospective clinical chart review was completed on 228 patients who delivered either Term or PTB (≥40 or <37 weeks gestation respectively) at Mayo Clinic Rochester. Paraffin-embedded placentas were obtained for analysis of immune cells (i.e. Hofbauer cells - HBCs), by imaging mass cytometry (IMC) and immunohistochemistry (IHC). Fresh biopsies of placenta and cell suspensions of cytotrophoblasts (CTBs) and HBCs from uncomplicated term pregnancies were exposed to inflammatory inducers (lipopolysaccharide LPS and polyI:C PIC) with production/secretion of inflammatory cytokines measured by ELISA and flow cytometry. One-way ANOVA, chi-squared, or Fisher’s exact test were conducted as appropriate. Results: Representative patients were identified based on covariates from in-depth chart review with equal number of male and female fetuses. To assess what immune cells are in the placenta, IMC analysis on representative patients found 22 distinct cell populations including CD163+CD68+ HBCs as the only immune cell detected within placenta villi (with some T cells and monocytes detected in the maternal space only) and a higher cell diversity in PTB compared to Term samples. To assess any differences in the number of HBCs in the placenta, IHC analysis on the whole cohort found significantly elevated number of CD68+CD163+ HBCs in placentas from PTB, regardless of labor classification, vs. Term. In explant models, exposure to LPS and PIC significantly increased the secretion/production of pro-inflammatory cytokines (IL-1β, IL-6, MCP-1, TNFα). Cell suspensions experiments revealed that HBCs had significantly more IL-6 production compared to CTBs at basal levels. In LPS-stimulated conditions, IL-6 production was significantly increased in HBCs only, compared to the lack of significant upregulation in CTBs. Conclusion: PTB groups had increased number of HBCs compared to term groups regardless of labor classification, suggesting that labor itself does not drive the placental immune profile. Spatial analysis in placenta confirmed HBCs as the only immune cell. Placental analysis by explants and cell suspensions determined HBCs as the driver of the observed inflammatory profile. Future work will investigate interactions between HBCs and trophoblasts and their spatially resolved roles in prenatal inflammation.