Neonatal and Maternal Adverse Outcomes Among Low-Risk Parous Women at 39-41 Weeks of Gestation

OBJECTIVE: To compare the composite neonatal or maternal adverse outcome among low-risk, parous women at 39-41 weeks of gestation. METHODS: This was a retrospective cohort study using the U.S. vital statistics data sets (2012-2016). We evaluated low-risk parous women with nonanomalous singleton gestations who delivered at 39, 40, or 41 weeks of gestation (as reported in completed weeks, eg, 39 weeks includes 39 0/7-39 6/7 weeks of gestation). The primary outcome, the composite neonatal adverse outcome, included any of the following: Apgar score less than 5 at 5 minutes, assisted ventilation for longer than 6 hours, neonatal seizure, or neonatal mortality. The secondary outcome, the composite maternal adverse outcome, included any of the following: intensive care unit admission, blood transfusion, uterine rupture, or unplanned hysterectomy. We used multivariable Poisson regression analyses to estimate the association between gestational age and adverse outcome (using adjusted relative risks [aRR] and 95% CI). RESULTS: Of 19.9 million live births during the study period, 5.4 million (27.1%) met inclusion criteria. Among them, 54.4% delivered at 39 weeks of gestation, 35.7% at 40 weeks, and 9.9% at 41 weeks. The overall rate of the composite neonatal adverse outcome was 4.86 per 1,000 live births. The risk of the composite neonatal adverse outcome was higher for those delivered at 40 (aRR 1.18; 95% CI 1.15-1.22) and 41 (aRR 1.59; 95% CI 1.53-1.65) weeks of gestation when compared with 39 weeks. The overall rate of the composite maternal adverse outcome was 2.31 per 1,000 live births. The risk of the composite maternal adverse outcome was also significantly higher with delivery at 40 (aRR 1.15; 95% CI 1.11-1.19) and 41 weeks of gestation (aRR 1.50; 95% CI 1.42-1.58) than at 39 weeks. CONCLUSION: Though only modestly, the rates of the composite neonatal and maternal adverse outcomes increase, from 39 through 41 weeks of gestation, among low-risk parous women.