Purpose <p>Socioeconomic status (SES) and body mass index (BMI) are risk factors for postpartum psychiatric episodes (PPE), but their relative contributions remain unclear. We examined how SES and BMI jointly influence PPE risk and whether BMI mediates the SES-PPE association across strata of psychiatric history.</p> Methods <p>We conducted a register-based cohort study of 995,513 childbirths (2004-2021). SES was defined by an index of income, education, and employment, and pre-pregnancy BMI was grouped according to WHO criteria. PPE was defined as a psychiatric diagnosis or two redeemed prescriptions for psychotropic medications within 12 months postpartum. Psychiatric history was stratified as none, past, or recent. Logistic regression estimated associations between SES, BMI, and PPE by psychiatric history. Causal mediation analysis assessed BMI’s mediational role in the SES-PPE association stratified by psychiatric history, and population attributable risk percentage (PARP) quantified the proportion of cases attributable to SES and BMI.</p> Results <p>Lower SES and BMI outside the normal range were independently associated with increased PPE risk, with effect sizes varying by psychiatric history. BMI mediated 1.26%-4.99% of the SES-PPE association, except for women with recent psychiatric history (21.37%). PARPs showed that SES accounted for a larger fraction of PPE cases (up to 31.40%) than BMI (4.48%-7.72%).</p> Conclusion <p>Lower SES and BMI outside the normal ranges increase PPE risk, but SES has a stronger population-level impact than BMI. The SES-PPE association is largely independent of BMI, indicating that factors related to social disadvantage – not BMI – drive most of the excess risk.</p>