Association between preterm birth subtypes and severe maternal morbidity: a retrospective cohort study

Background: Severe maternal morbidity (SMM) is a significant complication associated with preterm delivery. However, most studies have focused solely on SMM during the delivery hospitalization, without differentiating by preterm birth subtype or examining postpartum SMM and readmissions. Objective: To examine the association between preterm birth subtypes and SMM during delivery hospitalization, up to 1-year postpartum, and postpartum readmissions within 365 days. Study design: We conducted a retrospective cohort study using linked birth and fetal death certificates and maternal hospital discharge data from Michigan, Oregon, and South Carolina (2008–2020). Modified Poisson regression models were used to estimate adjusted relative risks (aRR) and 95% confidence intervals (CI) for SMM during delivery hospitalization, postpartum SMM, and readmissions, stratified by preterm birth subtype. Models were adjusted for birth year, state, insurance type, education, maternal age, race/ethnicity, and adequacy of prenatal care. Results: Among 2,570,808 deliveries, 156,763 (6.1%) were spontaneous preterm births and 70,877 (2.8%) were medically indicated preterm births. For medically indicated preterm births, the aRR for SMM during delivery hospitalization were 18.0 (95% CI 16.7–19.4) at ≤ 31 weeks, 14.5 (95% CI 13.2–15.8) at 32–33 weeks, and 6.7 (95% CI 6.3–7.1) at 34–36 weeks. For spontaneous preterm births, the corresponding aRRs were 7.5 (95% CI 7.0–8.0), 6.4 (95% CI 5.9-7.0), and 3.1 (95% CI 2.9–3.3), respectively. During the postpartum period, elevated risks of SMM persisted for both subtypes. For medically indicated preterm births, aRRs for postpartum SMM ranged from 3.4 (95% CI 3.0-3.9) at ≤ 31 weeks to 2.1 (95% CI 2.0-2.3) at 34–36 weeks. For spontaneous preterm births, aRRs for postpartum SMM ranged from 2.0 (95% CI 1.8–2.2) at ≤ 31 weeks to 1.6 (95% CI 1.5–1.7) at 34–36 weeks. Postpartum readmission risk was also elevated for both subtypes. Conclusion: Preterm delivery, particularly when medically indicated, is strongly associated with increased risk of SMM during delivery hospitalization, throughout the postpartum period, and with postpartum readmissions. These findings suggest that individuals experiencing preterm birth, especially medically indicated preterm deliveries, may benefit from targeted postpartum monitoring and follow-up, including earlier clinical contacts and closer surveillance for complications beyond the standard 6-week postpartum visit.