Labor induction at term and cesarean delivery rates - Earlier rather than later?

Objective
Mechanisms involved in parturition evolve throughout pregnancy, and therefore might be insufficiently mature for medically indicated induction of labor (IOL) during the different stages of pregnancy. We investigated labor induction failure rates (i.e. leading to cesarean delivery-CD) during the different stages of pregnancy.

Study Design
A population-based retrospective cohort analysis was performed, including all labor induction cases occurring at 32 to 44 gestational weeks, between 1991-2017, at a regional tertiary medical center. Congenital malformations, antepartum fetal demise, and multiple pregnancies were excluded. Rates of CD following IOL were assessed at different gestational weeks (GW). Term deliveries (≥37 weeks) following IOL were subdivided into separate GW. All post-term deliveries (≥42 weeks) following IOL were grouped together. A multivariable analysis was performed, assessing the independent association between IOL gestational age (within term) and failure rates, while adjusting for confounders.

Results
T The study population included 26,722 pregnancies involving IOL, 25,392 (95%) of which occurred at term. The trendline of CD rates according to gestational week demonstrated an "inverted J” shape, with a gradual decrease in CD rates as GA increased from 32 to 41 weeks (Figure, p< 0.001). A subsequent increase in CD rates was observed from 41 weeks and on. Among term pregnancies involving IOL, 73% were induced mechanically and the rest using prostaglandins, with a total CD rate of 16.8%. In the multivariate analysis, post-term IOL was independently associated with a higher CD rate (18.8%) as compared to IOL performed at 39 weeks’ gestation (17.4%, aHR 1.15, p=0.05, 95%CI 1.00-1.32, Table) while controlling for maternal age, IOL indication, previous CD, IOL method (mechanical vs. pharmacological) and birthweight.

Conclusion
In this large cohort of IOL cases, CD rates are relatively low, and decrease as GA increases. Post-term deliveries involving IOL are independently associated with a higher CD risk as compared to deliveries following IOL at 39 weeks' gestation.