The purpose of this study was to exami ne maternal and neonatal outcomes in relation to len gthening intervals of the second stage of labor.This is a retro spective cohort study of15,759nulliparous,term,cephalic,singleton births at the University of California,San Francisco,between 1976and2001.The second stage of labor was di vided into 1-hour intervals.Maternal and neonatal ou tcomes were compared with the use of chi -squared and Student t tests,and a probability value of≤.05was used to indicate statistical significance.Potential confounders were controlled for withmultivariate logistic regression.Increasing rates of cesarean delivery,operative vaginal delive ry,and perineal trauma were associated with the second stag e beyond the first hour.In multivariate analysis,the>4-hour interval group had higher rates of cesarean delivery(odds ratio,5.65;P <.001),operative vaginal deliveries(odds ratio,2.83;P <.001),3rd -or 4th -degree perineal lacera-tions(odds ratio,1.33;P =.009),and chorioam-nionitis(odds ratio,1.79;P <.001).There were no differences in neonatal acid -base s tatus associated with length of second stage.However,there were fewer neonates with a 5-minute Apgar score of<7(odds ratio,0.45;P =.01).Although the length of the second stage of labor is not associated with poor n eonatal outcome,a prolonged second stage is associate d with increased mater-nal morbidity and operative delivery rates. The purpose of this study was to exami ne maternal and neonatal outcomes in relation to len gthening intervals of the second stage of labor. This is a retro spective cohort study of 15,759 nulliparous, term, cephalic, singleton births at the University of California, San Francisco, between 1976 and 2001. The second stage of labor was di vided into 1-hour intervals. Maternal and neonatal ou tcomes were compared with the use of chi-squared and Student t tests, and a probability value of <0.05 was used to indicate statistical significance.Potential confounders were controlled for with multivariate logistic regression. Increasing rates of cesarean delivery, operative vaginal delive ry, and perineal trauma were associated with the second stag e beyond the first hour. multivariate analysis, the> 4-hour interval group had higher Rates of cesarean delivery (odds ratio, 5.65; P <.001), operative vaginal deliveries (odds ratio, 2.83; P <.001), 3rd-rth 4th-de gree perineal lacerations (odds ratio, 1.33; 009), and chorio There were no differences in neonatal acid-base s tatus associated with length of second stage. There was fewer neonates with a 5-minute Apgar score of <7 ( odds ratio, 0.45; P = .01) .Although the length of the second stage of labor is not associated with poor n eonatal outcome, a prolonged second stage is associate d with increased mater-nal morbidity and operative delivery rates.