METHODS: This cohort study, using the U.S. vital statistics datasets (2011–2015), evaluated low-risk nulliparous women with nonanomalous singleton gestations who labored at 39, 40, or 41 weeks of gestation (as reported in completed weeks of gestation; eg, 39 weeks include 39 0/7 to 39 6/7 weeks). The primary outcome, composite neonatal morbidity, included any of the following: Apgar score below 5 at 5 minutes, assisted ventilation longer than 6 hours, seizure, or mortality. The secondary outcome, composite maternal morbidity, included any of the following: intensive care unit admission, blood transfusion, uterine rupture, or unplanned hysterectomy. Multivariable Poisson regression was used to estimate the association between gestational age and morbidity (using adjusted relative risk [aRR] and 95% CI).

RESULTS: Of 19.8 million live births during the study interval, 3.3 million met inclusion criteria: 43.5% were delivered at 39 weeks of gestation, 41.4% at 40 weeks, and 15.1% at 41 weeks. The overall rates of composite neonatal and maternal morbidity were 8.8 and 2.8 per 1,000 live births, respectively. Composite neonatal morbidity was higher for those delivered at 40 (aRR 1.22; 95% CI 1.19–1.25) and 41 (aRR 1.53; 95% CI 1.49–1.58) weeks of gestation when compared with 39 weeks. Composite maternal morbidity was also significantly higher with delivery at 40 (aRR 1.19; 95% CI 1.14–1.25) and 41 weeks of gestation (aRR 1.56; 95% CI 1.47–1.65).

CONCLUSION: Among low-risk nulliparous women, the rate of composite neonatal and maternal morbidity increases, albeit modestly, from 39 through 41 weeks of gestation.