Heterogeneity of Preterm Birth Subtypes in Relation to Neonatal Death

OBJECTIVE: To investigate the heterogeneity of preterm labor, preterm premature rupture of membranes (PROM), and indicated preterm birth in overall and gestational-age-specific neonatal death risk. METHODS: We used 2001 U.S. linked birth/infant death (birth cohort) data sets for this analysis. We categorized three preterm birth subtypes according to reported preterm PROM, induction of labor, cesarean delivery, and pregnancy and labor complications. We used Cox proportional hazard models to calculate covariates adjusted hazard ratios (HRs) for neonatal death (0-27 days of life) among preterm neonates born at 24-27, 28-31, 32-33, and 34-36 weeks of gestation, with preterm labor being the referent. RESULTS: There were 3,763,306 singleton live births at 24-44 weeks of gestation in the data set. Preterm PROM, indicated preterm birth, and preterm labor had neonatal death risk of 2.7%, 1.8%, and 1.1%, respectively. Compared with preterm labor, preterm PROM had shorter gestational age and lower birth weight, so did indicated preterm birth but to a lesser extent. Preterm PROM and indicated preterm birth after 28 weeks of gestation were associated with higher neonatal death risk than preterm labor. At 34-36 weeks of gestation, the HR of preterm PROM was 1.53 (95% confidence interval 1.20-1.95), and the HR of indicated preterm birth was 2.06 (95% confidence interval 1.83-2.33). The increased risk from preterm PROM and indicated preterm birth was not limited to early neonatal death in the first 7 days. CONCLUSION: Preterm PROM and indicated preterm birth had higher risk of neonatal death than preterm labor, indicating heterogeneity in gestational age distribution and gestational-age-specific neonatal death risk. (Obstet Gynecol 2009;114:516-22)