Effect of early amniotomy on the risk of dystocia in nulliparous women. The Canadian Early Amniotomy Study Group.

Abstract

Early amniotomy has been advocated as a means of preventing dystocia, but its efficacy has not been studied prospectively. The purpose of this multicenter study was to determine whether routine early amniotomy reduces the risk of dystocia for nulliparous women in spontaneous labor. We studied 925 nulliparous women in labor, who were stratified according to the degree of cervical dilatation (< 3 cm vs. > or = 3 cm) and randomly assigned to either early rupture of the membranes (amniotomy group) or conservative management of labor (conservative-management group). Dystocia was defined as a period of at least four hours after dilatation of the cervix to 3 cm had been reached during which the mean rate of cervical dilatation was less than 0.5 cm per hour. Dystocia was significantly less frequent in the amniotomy group than in the conservative-management group (34 percent vs. 45 percent; relative risk, 0.8; 95 percent confidence interval, 0.6 to 0.9). The median length of time from randomization to full dilatation was 136 minutes shorter in the amniotomy group, and there was a trend toward less frequent use of oxytocin among the women assigned to amniotomy (36 percent vs. 41 percent; relative risk, 0.9; 95 percent confidence interval, 0.8 to 1.0). In a stratified analysis, the frequency of dystocia associated with amniotomy was reduced only among women with > or = 3 cm initial dilatation. The cesarean-section rate was similar in the two groups (amniotomy, 12 percent; conservative management, 11 percent). There were no statistically significant differences in outcome between the infants delivered by the women in the two groups; the measures of an adverse outcome included admission to a neonatal intensive care unit, five-minute Apgar score below 7, and arterial cord-blood pH below 7.2. Early amniotomy is an effective method of shortening the duration of labor and reducing the frequency of dystocia among nulliparous women in labor, but it does not lower the rate of cesarean section.