The impact of a previous cesarean delivery on in vitro fertilization outcomes: a registry-based cohort study

BACKGROUND: Cesarean delivery rates have climbed over the past few decades. We sought to evaluate whether a previous cesarean delivery is associated with reduced live birth rates following a subsequent embryo transfer when compared with patients with a history of vaginal delivery. OBJECTIVE: This study aimed to evaluate the impact of a previous cesarean delivery by reporting the live birth rate following the first embryo transfer and comparing it with the rates among patients with a previous vaginal delivery. STUDY DESIGN: We performed a retrospective, registry-based cohort study of all patients in Ontario, Canada, with a previous delivery, either vaginal or cesarean delivery, who subsequently had an embryo transfer between January 1, 2013, and December 31, 2020. A total of 7460 patients were included of which 4587 patients were in the vaginal delivery group and 2873 patients were in the cesarean delivery group. Data from the provincial birth register, Better Outcomes Registry & Network Ontario, and from the fertility registry, Canadian Assisted Reproductive Technology Register Plus were used for this study. A modified Poisson regression model was used to estimate the relative risk with 95% confidence intervals. Models were adjusted for age at oocyte retrieval, age at embryo transfer, number of previous deliveries, number of embryos transferred, type of embryo transfer (fresh vs frozen-thawed), and embryo stage at transfer. Crude risk differences with 95% confidence intervals were calculated. RESULTS: Patients with a previous cesarean delivery had a statistically significant lower live birth rate (adjusted risk ratio, 0.84; 95% confidence interval, 0.80-0.90; risk difference, -4.6 per 100; 95% confidence interval, -6.8 to -2.4), positive human chorionic gonadotropin rate (adjusted risk ratio, 0.92; 95% confidence interval, 0.88-0.96), implantation rate (adjusted risk ratio, 0.91; 95% confidence interval, 0.86-0.96), clinical pregnancy rate (adjusted risk ratio, 0.90; 95% confidence interval, 0.85-0.95), ongoing pregnancy rate (adjusted risk ratio, 0.87; 95% confidence interval, 0.82-0.92), and live birth rate with good perinatal outcome (adjusted risk ratio, 0.82; 95% confidence interval, 0.76-0.88) when compared with patients with a previous vaginal delivery. CONCLUSION: A previous cesarean delivery was associated with lower live birth rates and other fertility-related pregnancy outcomes after the first subsequent embryo transfer when compared with a previous vaginal delivery. Because residual confounding cannot be excluded in this retrospective, registry-based study, these findings should be interpreted as associations. The results of this study emphasize the importance of further research to investigate possible etiologies that underlie this association.