Impact of cesarean section on intermediate and late preterm births: United States, 2000-2003.

BACKGROUND: Cesarean section appears to be associated with increased risk of neonatal mortality among infants of low-risk term pregnancies, but it may offer some survival advantage among the most extremely preterm infants. The impact on intermediate (32-33 wk) and late preterm (34-36 wk) deliveries remains uncertain. The objective of this analysis was to compare the neonatal mortality rate (death at 0-27 days), the mechanical ventilation usage rate, and the incidence of hyaline membrane disease among intermediate and late preterm infants delivered by primary cesarean section compared with those delivered vaginally.

METHODS: United States Linked Birth and Infant Death Certificate files from the years 2000 to 2003 were used. Maternal demographic characteristics, medical complications, and labor and delivery complications were abstracted from the files along with infant information. Because of concern for misclassification of gestational age, a procedure was used to trim away births in which the birthweight of an infant for a specific gestational age was inconsistent. Adjusted odds ratios were calculated using logistic regression for the risk of the three outcomes of interest relative to the mode of delivery.

RESULTS: A total of 422,001 live births were available with complete data from the trimmed data set (60% of untrimmed data). After adjustment by logistic regression for infant size at birth, birthweight, sex, Apgar score at 5 minutes less than 4, multiple births, breech presentation, presence of an anomaly, the presence of any maternal medical condition or complication of labor and delivery, labor induction, maternal race, age, education, and gravidity, the adjusted odds ratios (95% CI for neonatal mortality at gestational ages of 32, 33, 34, 35, and 36 wk) were, respectively, 1.69 (1.31-2.20), 1.79 (1.40-2.29), 1.08 (0.83-1.40), 2.31 (1.78-3.00), and 1.98 (1.50-2.62).

CONCLUSIONS: These data suggest that for low-risk preterm infants at 32 to 36 weeks' gestation, independent of any reported risk factors, primary cesarean section may pose an increased risk of neonatal mortality and morbidity.