Objective: To evaluate whether extremely prolonged second stage of labor in nulliparous women affects mode of delivery and perinatal outcomes.Methods: We performed a retrospective cohort study of nulliparous women with singleton gestations and cephalic presentation who reached 10 cm of cervical dilation at gestational age 36 0/7-41 6/7. Women were stratified by epidural status. Deliveries were compared by length of second stage: 0-179 min (normal second stage, NSS), 180-299 min (prolonged second stage, PSS), and ≥300 min (extremely prolonged second stage, EPSS). Primary outcome was incidence of vaginal delivery. Secondary outcomes were maternal and neonatal morbidities.Results: Six hundred sixty-one women were evaluated; overall, 92.7% (613/661) of the patients delivered vaginally, with 84.6% (559/661) of women undergoing spontaneous vaginal delivery. In women with epidural anesthesia, 90.6% (446/492) delivered vaginally (97.2% of NSS, 95.1% of PSS, and 69.2% of EPSS). In women without epidural anesthesia, 98.8% (167/169) delivered vaginally (99.3% of NSS, 100.0% of PSS, and 87.5% of EPSS). Women with epidural anesthesia and EPSS had a higher rate of postpartum hemorrhage (aOR: 8.52; 95% CI: 3.99-18.19) and third-degree laceration when compared to NSS (aOR: 5.87; 95% CI: 1.71-20.17). EPSS neonates had a higher rate of CPAP use (OR: 3.99; 95% CI: 1.82-8.74) and significantly higher birth weight (p < .0001) and composite neonatal outcomes (OR: 4.98; 95% CI: 2.34-10.59) compared to NSS.Conclusion: In nulliparous women at term with singleton gestations who reached second stage, the chance of vaginal delivery was 92.7%; even after 5 h of second stage, most women delivered via vaginal delivery (70.4%, 81/115). In those with epidural anesthesia, the PSS group had similar perinatal outcomes as the NSS group, whereas the EPSS group had significantly worse perinatal outcomes. Second stage ≥5 h is a potential tipping point for hazardous perinatal outcomes.

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