Differentiating Preterm Birth Subtypes for Improved Maternal Care Planning

A new retrospective cohort study found that medically indicated preterm births carry substantially higher adjusted relative risks (aRRs) for severe maternal morbidity (SMM) than spontaneous preterm births, and that elevated SMM risk persists into the postpartum period.

This cohort included 2,570,808 deliveries in three U.S. states from 2008–2020. The team measured SMM during delivery hospitalization, through 1 year postpartum, and for postpartum readmissions. Outcomes were modeled with modified Poisson regression and reported as adjusted relative risks after controlling for demographics, insurance, prenatal care adequacy, and key comorbidities.

Medically indicated preterm births had larger aRRs for SMM than spontaneous preterm births across gestational strata: indicated aRRs 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, whereas spontaneous aRRs were 7.5 (95% CI 7.0–8.0) at ≤31 weeks, 6.4 (95% CI 5.9–7.0) at 32–33 weeks, and 3.1 (95% CI 2.9–3.3) at 34–36 weeks. These differences indicate that underlying maternal conditions prompting medically indicated delivery account for much of the excess SMM risk relative to spontaneous preterm birth.

Elevated SMM persisted into the postpartum period for both subtypes, with postpartum estimates higher after indicated preterm birth than after spontaneous preterm birth: postpartum aRRs for medically indicated deliveries 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, while spontaneous postpartum aRRs 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 followed the same pattern as gestational age fell; the persistence of elevated postpartum SMM highlights sustained vulnerability after preterm delivery and supports intensified postpartum monitoring for recent preterm births.

Translating these findings into surveillance logic suggests stratifying antenatal surveillance intensity by preterm birth subtype and maternal comorbidity profile—prioritizing closer monitoring and earlier detection of hypertensive disorders and hemorrhage risk in those with medically indicated preterm delivery histories—and documenting planned postpartum follow-up for all patients who deliver preterm. Subtype-aware risk stratification aligns surveillance resources with the highest-yield patients and clarifies which antenatal signals warrant escalated evaluation.