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Mitha A, Chen R, Altman M, Johansson S, Stephansson O, Bolk J. Neonatal morbidities in infants born late preterm at 35-36 weeks of gestation - a Swedish nationwide population-based study. J Pediatr. 2021 Mar 1:S0022-3476(21)00205-5. doi: 10.1016/j.jpeds.2021.02.066. Epub ahead of print. PMID: 33662344.

Objective: To assess risk for neonatal morbidities among infants born late preterm at 35-36 gestational weeks, early-term (37-38 weeks), and late-term (41 weeks) infants, compared with full-term (39-40 weeks) infants.

Study design: This nationwide population-based cohort study included 1,650,450 non-malformed liveborn singleton infants born at 35 to 41 weeks between 1998 and 2016 in Sweden. Risk ratios (RRs) for low Apgar score (0-3) at 5 minutes; respiratory, metabolic, infectious and neurological morbidities; and severe neonatal morbidity (composite outcome) were adjusted for maternal, pregnancy, delivery and infant characteristics.

Results: Compared with infants born at 39-40 weeks, adjusted RRs and proportions of infants born at 35-36 weeks were higher for: metabolic morbidity 7.79 (7.61 to 7.97), (33.75% versus 3.11%); respiratory morbidity 5.54 (5.24 to 5.85), (5.49% versus 0.75%); severe neonatal morbidity 2.42 (2.27 to 2.59), (3.40% versus 1.03%); infectious morbidity 1.98 (1.83 to 2.14), (2.53% versus 0.95%); neurological morbidity 1.74 (1.48 to 2.03), (0.54% versus 0.23%); and low Apgar score 2.07 (1.72 to 2.51), (0.42% versus 0.12%). Risks for respiratory, severe neonatal morbidity, infectious, neurological morbidities, and low Apgar score were highest at 35 weeks, gradually declined until 39 weeks, and increased at 39 to 41 weeks.

Conclusions: Infants born late preterm at 35-36 weeks of gestation are at increased risk of neonatal morbidities, although absolute risks for severe neonatal morbidities are low. Our findings reinforce the need of preventing late preterm delivery to reduce the burden of neonatal morbidity, and help professionals and families with a better risk assessment.

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