What is the Best Mode of Delivery in Nulliparous, Singleton, Term, Vertex Pregnancies of Individuals ≥ 35 Years Old?: Short Title: What is the Best Mode of Delivery in Nulliparous, Singleton, Term, Vertex Pregnancies?

With approximately 145 million births occurring worldwide each year - over 30 million by cesarean delivery, the need for evaluation of maternal and perinatal outcomes in different delivery scenarios is more pressing than ever. Recently, in a meta-analysis of the available randomized controlled trials (RCTs), planned cesarean delivery was associated with decreased rates of low umbilical artery pH, and neonatal complications such as birth trauma, tube feeding, and hypotonia when compared to planned vaginal delivery. Among singleton pregnancies, planned cesarean delivery was associated with a lower rate of perinatal death. For mothers, planned cesarean delivery was associated with significantly less chorioamnionitis, more wound infection, and less urinary incontinence at 1-2 years. Conversely, planned vaginal delivery has been associated with benefits such as a lower incidence of wound infection and quicker postpartum recovery compared to planned cesarean delivery. Nonetheless, several risk factors for cesarean delivery are increasing - such as older maternal age, obesity, diabetes, excessive gestational weight gain, and birth weight - while maternal pelvises are getting smaller. Concerns about the potential long-term risks of multiple cesarean deliveries, such as placenta accreta spectrum disorders, highlight the need for a balanced evaluation of both delivery modes. However, the total fertility rate is decreasing in the US and around the world, with many people wanting two or fewer babies, which decreases future risk of placenta accreta incurred by multiple cesarean deliveries in these individuals. Furthermore, one in four obstetricians-gynecologists has undergone a cesarean delivery on maternal request for their nulliparous, singleton, term, vertex (NSTV) pregnancy, and cesarean delivery rates less than about 19% have been associated with higher perinatal and maternal mortality. Thus, we propose that it is imperative that we prioritize conducting randomized trials to compare planned cesarean to planned vaginal delivery for NSTV pregnancies. Such trials would need to include 8,000 or more individuals; they would ideally follow each participant to the end of their reproductive life and study perinatal and maternal outcomes, including non-biologic outcomes such as patient satisfaction, postpartum depression, breastfeeding rates, mother-infant bonding, post-traumatic stress, and cost-effectiveness. The time for such a trial is now, as it holds the potential to inform and improve obstetrical care practices globally.