Objective: To analyze neonatal mortality and morbidity rates at 34, 35, and 36 weeks of gestation compared with births at term over the past 18 years at our hospital and to estimate the magnitude of increased risk associated with late preterm births compared with births later in gestation.
Methods: We performed a retrospective cohort study of births at our hospital over the past 18 years. The study included all liveborn singleton infants between 34 and 40 weeks of gestation and without anomalies that were delivered to women who received prenatal care in our hospital system. Neonatal outcomes for late preterm births were compared with those for infants delivered at 39 weeks.
Results: Late preterm singleton live births constituted approximately 9% of all deliveries at our hospital and accounted for 76% of all preterm births. Late preterm neonatal mortality rates per 1,000 live births were 1.1, 1.5, and 0.5 at 34, 35, and 36 weeks, respectively, compared with 0.2 at 39 weeks (P<.001). Neonatal morbidity was significantly increased at 34, 35, and 36 weeks, including ventilator-treated respiratory distress, transient tachypnea, grades 1 or 2 intraventricular hemorrhage, sepsis work-ups, culture-proven sepsis, phototherapy for hyperbilirubinemia, and intubation in the delivery room. Approximately 80% of late preterm births were attributed to idiopathic preterm labor or ruptured membranes and 20% to obstetric complications.
Conclusion: Late preterm births are common and associated with significantly increased neonatal mortality and morbidity compared with births at 39 weeks. Preterm labor was the most common cause (45%) for late preterm births.
Level of evidence: II.