B4-1. Primary aim Primary outcome measure: The primary outcome is spontaneous vaginal delivery, defined as delivery that occurs without the use of forceps, vacuum, or cesarean, as the primary outcome measure. This is because spontaneous vaginal delivery is the most desirable outcome for laboring women in the second stage of labor. Compared to cesarean delivery, spontaneous vaginal delivery is associated with lower maternal and neonatal morbidity, including hemorrhage, postoperative wound infection, endometritis, and prolonged hospitalization. Further, once a cesarean is performed, the risk of delivery by cesarean for future pregnancies is dramatically elevated, further increasing maternal morbidity. Finally, although operative vaginal delivery is associated with overall lower maternal morbidity than cesarean delivery, it carries higher risks of neonatal and maternal pelvic floor injury than spontaneous vaginal delivery.