Induction of labor and cesarean birth in lower-risk nulliparous women at term: A retrospective cohort study

Objective: To evaluate whether induction of labor (IOL) is associated with cesarean birth (CB) and perinatal mortality in uncomplicated first births at term compared with expectant management outside the confines of a randomized controlled trial. Methods: Population-based retrospective cohort study of all births in Victoria, Australia, from 2010 to 2018 (n = 640,191). Preliminary analysis compared IOL at 37 weeks with expectant management at that gestational age and beyond for uncomplicated pregnancies. Similar comparisons were made for IOL at 38, 39, 40, and 41 weeks of gestation and expectant management. The primary analysis repeated these comparisons, limiting the population to nulliparous women with uncomplicated pregnancies and excluding those with a medical indication for IOL. We compared perinatal mortality between groups using Chi-square tests and multivariable logistic regression for all other comparisons. Adjusted odds ratios and 99% confidence intervals were reported. p < 0.01 denoted statistical significance. Results: Among nulliparous, uncomplicated pregnancies at >= 37 weeks of gestation in Victoria, IOL increased from 24.6% in 2010 to 30.0% in 2018 (p < 0.001). In contrast to the preliminary analysis, the primary analysis showed that IOL in lower-risk nulliparous women was associated with increased odds of CB when performed at 38 (aOR 1.23(1.13-1.32)), 39 (aOR 1.31(1.23-1.40)), 40 (aOR 1.42(1.35-1.50)), and 41 weeks of gestation (aOR 1.43(1.35-1.51)). Perinatal mortality was rare in both groups and non-significantly lower in the induced group at most gestations. Discussion: For lower-risk nulliparous women, the odds of CB increased with IOL from 38 weeks of gestation, along with decreased odds of perinatal mortality at 41 weeks only.