Amniotomy is usually used for induction or augmentation of labor and if timely applied it could influence the labor promotion. Objective: The aim of the present study was to evaluate the time of amniotomy on labor course. Methods: This prospective randomized clinical trial was carried out at Kosar Hospital affiliated to Qazvin University of Medical Sciences (Iran) in 2005. Nulliparous women ≥ 40 weeks of a singleton gestation, cephalic presentation, intact membrane, and cervical dilatation ≤

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... were selected. All women underwent cervical ripening using a foley catheter. Following spontaneously removal of the catheter, women were randomly assigned to either early or late amniotomy. While in first group amniotomy was performed immediately, it was carried out in second group at active phase following the beginning of oxytocin infusion. Duration of labor, cesarean section rate and indications, and Apgar score at 5 minutes in two groups were recorded. Data were analyzed using statistical tools including t test and chi square test. Findings: Duration of labor from the time of catheter removal to delivery in early amniotomy was longer than that of late amniotomy [12 hours and 40 minutes versus 9 hours and 40 minutes, (P=0.35)]. The rate of cesarean section was shown to be higher in early amniotomy group compared to late amniotomy group (27.3% versus 14.5% with a relative risk value of 1.88). An increase in rate of cesarean section was primarily due to dystocia (52.4% versus 16.4% and a relative risk of 1.44) however, no significant difference was found in rate of cesarean section due to others indications. Also, the difference between Apgar score at 5 minutes among two groups was shown to be insignificant, statistically (P=0.15). Conclusion: In women undergoing cervical ripening using a foley catheter, the augmentation of labor by oxytocin followed by amniotomy during active phase of labor results in shortening of duration of labor and also lower rate of cesarean section for dystocia.