Late-preterm birth: does the changing obstetric paradigm alter the epidemiology of respiratory complications?

OBJECTIVE: To analyze the effect of gestational age, delivery mode, and maternal-fetal risk factors on rates of respiratory problems among infants born 34 or more weeks of gestation over a 9-year period.
METHODS: Retrospective analysis of prospectively collected maternal and neonatal data on all inborn births at 34 or more weeks of gestation at a single tertiary care center for the years 1990-1998. Specific diagnostic criteria were concurrently applied by a single investigator.
RESULTS: Over the 9-year period, late-preterm births increased by 37%, whereas births at more than 40 weeks decreased by 39%, resulting in a decrease in median age at delivery from 40 weeks to 39 weeks (P<.001). Respiratory problems occurred in 705 term or late-preterm infants (4.9%), with clinically significant morbidity (respiratory distress syndrome, meconium aspiration syndrome, or pneumonia) least common at 39-40 weeks of gestation. Respiratory morbidity was greater among infants born by cesarean delivery or complicated vaginal delivery compared with uncomplicated cephalic vaginal delivery. The rate of respiratory morbidity did not change over time (1990-1992 1.3%, 1993-1995 1.5%, 1996-1998 1.4%, P=.746). The etiologic fraction for respiratory morbidity did not change over time for infants 34-36 weeks but decreased twofold for infants born after 40 weeks.
CONCLUSION: Over the 9-year study period, reduced respiratory morbidity associated with decreased births after 40 weeks were offset by the adverse respiratory effect of increased cesarean delivery rates and increased late-preterm birth rates.