Influence of Cesarean Section on Postpartum Fertility and Dysmenorrhea: A Retrospective Cohort Study in Japan.

Objective: To investigate the association between cesarean section (CS) and postpartum fertility and dysmenorrhea using data from a Japanese insurance registry. Methods: This retrospective cohort study used a data set of patients registered between 2007 and 2021 in an insurance registry comprising specific employee-based health insurance companies in Japan. Of those data sets, we included data from participants who had their first recorded childbirth between 2014 and 2018. The exclusion criteria were any prior deliveries, dysmenorrhea, or complications that would affect the next pregnancy or postpartum dysmenorrhea since 2007. The occurrence of subsequent childbirth and postpartum dysmenorrhea until 2021 was compared between the CS and vaginal delivery (VD) groups using the log-rank test and Cox proportional hazards model with stratification according to age and age matching. Results: This study included 25,984 (5,926 after age matching) and 5,926 participants in the VD and CS groups, respectively. After age matching, the rate of subsequent childbirth was 18.3% and 16.3%, and the rate of postpartum dysmenorrhea was 6.5% and 7.8% in the VD and CS groups, respectively. There were fewer subsequent childbirths in the CS group than in the VD group after age matching in the stratified Cox proportional hazards model (hazard ratio [HR] 95% confidence interval [CI]: 0.86 [0.79-0.94]). The CS group had a significantly higher risk of dysmenorrhea (HR [95% CI]: 1.18 [1.03-1.36]). Conclusions: Although confounding might be existing, our study suggests that CS might be associated with decreased postpartum fertility and increased dysmenorrhea. The medical indications for CS should be carefully determined; post-CS women should be meticulously followed up.