OBJECTIVE: To characterize precursors for late preterm birth in singletons and incidences of neonatal morbidities and perinatal mortality by gestational age and precursor. METHODS: Using retrospective observational data, we compared 15,136 gestations born late preterm with 170,593 deliveries between 37 0/7 and 41 6/7 weeks. We defined the following categories of precursors for late preterm delivery: "spontaneous labor," "premature rupture of the membranes (preterm PROM)," "indicated" delivery, and "unknown." Incidences of neonatal morbidities were calculated according to category of precursor stratified by gestational age at delivery. Neonatal morbidities and mortality associated with potentially avoidable deliveries (eg, "soft" precursors or elective) were compared between late preterm births and neonates born at 37-40 weeks. RESULTS: Late preterm birth comprised 7.8 of all births and 65.7 of preterm births. Percentages of precursors were 29.8 spontaneous labor, 32.3 preterm PROM, 31.8 "indicated" (obstetric, maternal, or fetal condition), and 6.1 unknown. Different precursors for delivery were associated with varying incidences of neonatal morbidity. One in 15 neonates delivered late preterm for "soft" or elective precursors, and neonatal morbidity and mortality were increased compared with delivery at or after 37 weeks for these same indications. CONCLUSION: A significant number of late preterm births were potentially avoidable. Elective deliveries should be postponed until 39 weeks of gestation. LEVEL OF EVIDENCE: II.
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