Subtypes of preterm birth and the risk of postneonatal death

Objective: To examine the differences in postneonatal death risk among 3 clinical subtypes of preterm birth: preterm premature rupture of membranes (PROM), indicated preterm birth, and spontaneous preterm labor. Study design: We analyzed the 2001-2005 US linked birth/infant death (birth cohort) datasets. The preterm birth subtypes were classified using information on the birth certificate: reported PROM, induction of labor, cesarean section, and complications of pregnancy and labor. Cox proportional hazard models were used to estimate covariate-adjusted hazard ratios and 95% CIs for postneonatal death (from days 28 to 365). Estimation was given for preterm birth subtypes in a week-by-week analysis. Causes of death were analyzed by preterm birth subtype and then separately at 24-27, 28-31, and 32-36 weeks of gestation. Results: For the total of 1 895 350 singleton preterm births who survived the neonatal period, the postneonatal mortality rate was 1.11% for preterm PROM, 0.78% for indicated preterm birth, and 0.53% for spontaneous preterm labor. Preterm PROM was associated with significantly higher risk of postneonatal death compared with spontaneous preterm labor in infants born at 27 weeks gestation or later. Similarly, indicated preterm birth was associated with a significantly higher risk of postneonatal death than spontaneous preterm labor in infants born at 25 weeks gestation or later. Preterm PROM and indicated preterm birth were associated with greater risk of death in the postneonatal period compared with spontaneous preterm labor, irrespective of the cause of death. Conclusion: Subtypes of preterm birth carry different risks of postneonatal mortality. Prevention of preterm-related postneonatal death may require more research into the root causes of preterm birth subtypes.