Likelihood of primary cesarean section following induction of labor in singleton cephalic pregnancies at term, compared with expectant management: An Australian population-based, historical cohort study.

Introduction: There has been increased use of both induction of labor (IOL) and cesarean section for women with term pregnancies in many high-income countries, and a trend toward birth at earlier gestational ages. Existing evidence regarding the association between IOL and cesarean section for term pregnancies is mixed and conflicting, and little evidence is available on the differential effect at each week of gestation, stratified by parity.

Material and methods: To explore the association between IOL and primary cesarean section for singleton cephalic pregnancies at term, compared with two definitions of expectant management (first: at or beyond the week of gestation at birth following IOL; and secondary: only beyond the week of gestation at birth following IOL), we performed analyses of population-based historical cohort data on women who gave birth in one Australian state (Queensland), between July 1, 2012 and June 30, 2018. Women who gave birth before 37⁺⁰ or after 41⁺⁶ weeks of gestation, had stillbirths, no-labor, multiple births (twins or triplets), non-cephalic presentation at birth, a previous cesarean section, or missing data on included variables were excluded. Four sub-datasets were created for each week at birth (37-40). Unadjusted relative risk, adjusted relative risk using modified Poisson regression, and their 95% confidence intervals were calculated in each sub-dataset. Analyses were stratified by parity (nulliparas vs. parous women with a previous vaginal birth). Sensitivity analyses were conducted by limiting to women with low-risk pregnancies.

Results: A total of 239 094 women were included in the analysis, 36.7% of whom gave birth following IOL. The likelihood of primary cesarean section following IOL in a Queensland population-based cohort was significantly higher at 38 and 39 weeks, compared with expectant management up to 41⁺⁶ weeks, for both nulliparas and paras with singleton cephalic pregnancies, regardless of risk status of pregnancy and definition of expectant management. No significant difference was found for nulliparas at 37 and 40 weeks; and for paras at 40 weeks.

Conclusions: Future studies are suggested to investigate further the association between IOL and other maternal and neonatal outcomes at each week of gestation in different maternal populations, before making any recommendation.