Perinatal outcomes in low-risk term pregnancies: do they differ by week of gestation?

Objective: The objective of the study was to examine whether the risk of perinatal complications increases with increasing gestational age among term pregnancies. Study Design: This is a retrospective cohort study of low-risk women with term, singleton births in 2003 in the United States. Gestational age was subgrouped into 37, 38, 39, 40, and 41 completed weeks. Statistical comparison was performed using χ² test and multivariable logistic regression models, with 39 weeks' gestation as the referent. Results: There were 2,527,766 women meeting study criteria. Compared with 39 weeks, delivery at 37 or 38 weeks had lower risk of febrile morbidity but slightly higher risk of cesarean delivery. Delivery at 40 or 41 weeks was also associated with higher overall maternal morbidity. For neonates, delivery at 40 or 41 weeks had higher risk of birthweight greater than 4500 g, neonatal injury (40 weeks: adjusted odds ratio [aOR] 1.11 [95% confidence interval (CI), 1.05-1.18]; 41 weeks: aOR 1.27 [95% CI, 1.17-1.37]) and meconium aspiration (40 weeks: aOR 1.55 [95% CI, 1.43-1.69]; 41 weeks: aOR 2.12 [95% CI, 1.91-2.35]). Delivery at 37 or 38 weeks had higher risk of hyaline membrane disease (37 weeks: aOR 3.12 [95% CI, 2.90-3.38]); 38 weeks: aOR 1.30 [95% CI, 1.19-1.43]) but lower risk of meconium aspiration. Conclusion: The risk of cesarean delivery and neonatal morbidity in low-risk women increases at 40 weeks and beyond, whereas the odds of serious neonatal pulmonary disease were highest at 37 weeks. Recognition of such variation in term outcomes should lead providers to avoid iatrogenic morbidity and consider interventions to prevent complications of late-term pregnancy.