BACKGROUND: Placenta Accreta Spectrum (PAS) disorders are associated with substantial maternal morbidity, and distinguishing mild from severe forms prenatally remains difficult. Inflammatory indices derived from peripheral blood counts have emerged as promising biomarkers, yet their role in grading PAS severity has been insufficiently investigated. This study aimed to assess the discriminative value of the systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), and other inflammatory indices for differentiating severe from mild PAS within an already diagnosed PAS population in late pregnancy. METHODS: This retrospective single-centre cohort study included 1,276 patients with histopathologically confirmed PAS. Third-trimester neutrophil, lymphocyte, monocyte, and platelet counts were used to derive neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), SII, and SIRI. Multivariable logistic regression, receiver operating characteristic curve analysis, and decision curve analysis (DCA) were conducted. RESULTS: Compared with the mild PAS group (n = 1,060), the severe PAS group (n = 216) showed significantly lower SIRI, SII, NLR, and PLR values (all P < 0.001). SIRI demonstrated moderate discriminative performance (area under the curve = 0.712). DCA suggested its potential clinical usefulness across threshold probabilities. CONCLUSIONS: Lower third-trimester SIRI, SII, NLR, and PLR values are associated with severe PAS. SIRI shows moderate diagnostic performance and may serve as a cost-effective adjunct to imaging for prenatal risk stratification. Prospective validation is warranted.