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OBJECTIVE: To estimate the effect of gestational age on short-term neonatal morbidity in cases of spontaneous, low-risk singleton late preterm deliveries and to identify predictors of adverse neonatal outcome.

METHODS: This was a retrospective study of all spontaneous, low-risk late preterm deliveries (34 0/7 to 36 6/7 weeks of gestation) during the years 1997 to 2006 (n=2,478). Multiple gestations and pregnancies complicated by preterm premature rupture of membranes (PROM) or maternal or fetal complications were excluded. Short-term neonatal outcome was compared with a control group of full-term deliveries in a 3:1 ratio (n=7,434). Logistic regression analysis was used to identify risk factors for neonatal morbidity among late preterm infants.

RESULTS: Compared with full-term infants, spontaneous late preterm delivery was independently associated with an increased risk of neonatal morbidity, including respiratory distress syndrome (4.2% compared with 0.1%, P<.001), sepsis (0.4% compared with 0.04%, P<.001), intraventricular hemorrhage (0.2% compared with 0.02%, P<.001), hypoglycemia (6.8% compared with 0.4%, P<.001), and jaundice requiring phototherapy (18% compared with 2.5%, P<.001). Cesarean delivery (odds ratio [OR] 2.1, 95% confidence interval [CI] 1.6-2.6), male sex (OR 1.4, 95% CI 1.1-1.8), and multiparity (OR 2.2, 95% CI 1.7-2.8) were independent risk factors for neonatal respiratory morbidity in cases of late preterm deliveries. The relationship between gestational age and neonatal morbidity was of continuous nature with a nadir at about 39 weeks rather than a term-preterm threshold phenomenon and was unrelated to birth weight.

CONCLUSION: Late prematurity is associated with significant neonatal morbidity in cases of spontaneous low-risk singleton deliveries. This information is important for appropriate counseling and should stimulate efforts to decrease the rate of late preterm deliveries.

LEVEL OF EVIDENCE: II

(C) 2009 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.