Active phase labor arrest: revisiting the 2-hour minimum

To generate contemporary uterine activity and labor progress data for oxytocin-augmented labor, and assess whether 2 hours of active phase labor arrest with at least 200 Montevideo units justifies cesarean delivery.

Five hundred and one consecutive spontaneously laboring term women with abnormally progressive labor were managed by a standardized protocol: oxytocin and intrauterine pressure catheter with an intent to sustain at least 200 Montevideo units for 4 hours or more before cesarean for labor arrest. Uterine activity was measured, and maternal and neonatal outcomes were evaluated. With a sample of this size, the upper 95% confidence interval limit for an event with an observed rate of 1% is below 3%.

During oxytocin augmentation, nulliparas who were delivered vaginally dilated at a median rate of 1.4 cm/hour versus 1.8 cm/hour for parous women. In both groups, the 5th percentile of cervical dilation rate was 0.5 cm/hour. Thirty-eight women experienced labor arrest for over 2 hours despite at least 200 sustained Montevideo units; 23 (61%) achieved a vaginal delivery. Rates of chorioamnionitis and endometritis for the 38 women were 26%. None of their infants sustained a serious complication, including brachial plexus injury, even though three of the 23 vaginal deliveries (13%) were complicated by shoulder dystocia.

These data demonstrate that oxytocin-augmented labor proceeds at substantially slower rates than spontaneous labor, and support our previous contention that the criteria of labor arrest for 2 hours, despite at least 200 sustained Montevideo units, are insufficiently rigorous for the performance of cesarean.