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Perinatal morbidity associated with late preterm deliveries compared to deliveries between 37–40 weeks



To estimate the risk of short-term complications in neonates born between 34 and 36 weeks by week of gestation.


This is a retrospective cohort study.


Deliveries in 2005 in the United States of America.


Singleton live births between 34 and 40 weeks gestational age.


Gestational age was subgrouped into 34, 35, 36 and 37–40 completed weeks. Statistical comparisons were performed using chi-square test and multivariable logistic regression models, with 37–40 weeks gestational age designated as referent.

Main Outcome Measures

Perinatal morbidities, including 5-minute Apgar scores, hyaline membrane disease, neonatal sepsis/antibiotics use, and admission to the intensive care unit.


There were 175,112 neonates born between 34 and 36 weeks in 2005. Compared to neonates born between 37 and 40 weeks, neonates born at 34 weeks had higher odds of 5-minute Apgar<7 (adjusted odds ratio [aOR]=5.51, 95% CI [5.16–5.88]), hyaline membranes disease (aOR=10.2 [9.44–10.9]), mechanical ventilation use >6 hours (aOR=9.78 [8.99–10.6]) and antibiotics use (aOR=9.00 [8.43–9.60]). Neonates born at 35 weeks were similarly at risk of morbidity, with higher odds of 5-minute Apgar <7 (aOR 3.42 [3.23–3.63], surfactant use (aOR 3.74 [3.21–4.22], ventilation use >6 hours (aOR 5.53 [5.11–5.99] and NICU admission (aOR 11.3 [11.0–11.7). Further, neonates born at 36 weeks remain at higher risk of morbidity compared to deliveries at 37–40 weeks.


While the risk of undesirable neonatal outcomes decreases with increasing gestational age, the risk of neonatal complications in late preterm births remains higher compared to infants delivered at 37–40 weeks gestation.

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