Background: Severe maternal morbidity (SMM) encompasses a spectrum of life-threatening conditions, with blood transfusion being the most frequent indicator. However, transfusion-related SMM may reflect hospital practice patterns rather than intrinsic maternal risk. This study examined the differential associations of prenatal care adequacy and socioeconomic status (SES) with SMM with and without blood transfusion to better identify high-risk maternal populations.

Methods: Using the Korean National Health Insurance Service database (2003–2021), we analyzed 7,171,578 deliveries. SMM was defined per the Centers for Disease Control and Prevention (CDC) algorithm, and further categorized into (1) SMM including transfusion and (2) SMM excluding transfusion. Adequate prenatal care (PNC) was classified using the Kessner Index; SES was based on income quartiles. Modified Poisson regression with robust error estimation estimated adjusted relative risks (aRRs) for each SMM type.

Results: Overall, 2.1% of deliveries involved SMM. Inadequate prenatal care was associated with significantly higher risk of SMM both with and without blood transfusion (with blood transfusion:  aRR = 1.16, 95% CI: 1.14–1.19; without blood transfusion: aRR = 1.20, 95% CI: 1.16-1.24). Women in the lowest income level receiving inadequate care had more than 30% higher risk for both SMM with transfusion (aRR = 1.35; 95% CI: 1.26–1.44) and without transfusion (aRR = 1.32, 95% CI: 1.28-1.38) compared to the highest-income level women with adequate PNC.

Conclusion: While both SMM types were associated with inadequate prenatal care and low income, the relative difference was smaller for non-transfusion SMM, likely reflecting its inherently severe nature and high baseline risk. Distinguishing transfusion-based from non-transfusion SMM can improve risk stratification and better guide targeted prenatal and perinatal interventions to reduce preventable maternal complications.