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Background 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.
Methods We studied 925 nulliparous women in labor, who were stratified according to the degree of cervical dilatation (<3 cm vs.
Results 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
Conclusions 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.
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. 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.
The results of some previous randomized trials2,3,4,5 but not all6 suggested that early amniotomy reduces the average duration of labor. Whether a policy of early amniotomy reduces the frequency of dystocia, however, is not known. In addition, earlier studies did not have adequate power to determine whether the rate of oxytocin administration or the rate of cesarean section was affected by early amniotomy. We undertook this study to determine whether routine early amniotomy for nulliparous women in spontaneous labor was associated with a reduction in the risk of dystocia. Secondary outcomes included the rates of cesarean section, oxytocin administration, abnormalities in the fetal heart rate, and indicators of maternal and neonatal morbidity.
Methods
This study was carried out in 11 university-affiliated teaching hospitals (10 in Canada and 1 in the United States) from October 1989 through April 1991. The annual numbers of births per hospital varied from approximately 2000 to 8000. Women who were admitted to the hospital in labor were eligible to participate in the study if they were nulliparous, at or beyond 38 weeks' gestation, and in spontaneous labor; if they had a single fetus in the cephalic presentation and intact membranes; and if the fetal heart rate was normal on the basis of auscultation or electronic monitoring. Women were excluded from the study if intrauterine growth retardation (estimated fetal weight below the fifth percentile on ultrasonography) was suspected, or if severe preeclampsia, insulin-dependent diabetes mellitus, or cervical dilatation 6 cm was present. Women whose distress was too great to permit us to obtain informed consent were excluded. The study was approved by the ethics committees of all participating institutions, and written consent was obtained from each woman.
Randomization took place when the woman had had painful uterine contractions every five minutes for a period of at least one hour and had either cervical dilatation of 3 cm or more or, if cervical dilatation was less than 3 cm, an increase in dilatation of at least 1 cm after admission to the hospital. In addition, the fetal head had to be firmly applied to the cervix.
Randomization was centralized by means of a telephone-answering service, and group assignment was stratified according to medical center and the degree of cervical dilatation at the time of the last examination before randomization (<3 cm dilatation vs. 3 cm dilatation). The women were assigned to one of two treatment groups. Those in the amniotomy group (n = 462) underwent artificial rupture of the membranes by a physician using a sterile plastic hook as soon as possible after randomization; for those assigned to the conservative-management group (n = 463), care givers attempted to avoid amniotomy unless there was a medical indication, such as the need for internal monitoring of the fetal heart rate, an arrest of cervical dilatation for at least two hours, or dystocia. Administration of oxytocin was permitted in both groups in the presence of dystocia defined according to the study criteria.
For this study, dystocia was defined according to the guidelines suggested by the panel of the National Consensus Conference on Aspects of Cesarean Birth (Canada)7. A diagnosis of dystocia was reserved for women in whom at least four hours elapsed after cervical dilatation had reached 3 cm, during which time the average rate of cervical dilatation was less than 0.5 cm per hour. Umbilical-cord arterial and venous blood samples were obtained separately at delivery for measurement of pH. Data on the progress of labor, the method of delivery, and indicators of maternal morbidity and neonatal status were obtained from the hospital record after discharge.
All available tracings of fetal heart rate were reviewed after delivery by a single obstetrician who was not aware of the woman's group assignment. A tracing was classified as abnormal if any of the following features were identified: tachycardia >160 beats per minute for more than 20 minutes; bradycardia <110 beats per minute for more than 10 minutes; persistent reduced long-term variability (<5 beats per minute) in combination with late decelerations ( 3); severe variable decelerations ( 3); and prolonged decelerations ( 1).
Statistical Analysis
Proportions were compared with the chi-square test or Fisher's exact test. We used t-tests for independent samples to compare means and Wilcoxon tests to compare medians. The relative risks and 95 percent confidence intervals8 are expressed with the conservative-management group as the reference group. Kaplan-Meier survival analysis was used to estimate the probability of a woman's continuing in labor according to the length of time from randomization,9 and the curves were compared with the log-rank test10. In this analysis, delivery was considered as a "failure."
Results
During the study period, approximately 19 percent of the 18,200 nulliparous women who gave birth in the participating hospitals were eligible to participate in the study. The most frequent reasons for exclusion were ruptured membranes at the time of hospitalization (43 percent), induction of labor (22 percent), gestational age less than 38 weeks (26 percent), cervical dilatation greater than 5 cm at admission (8 percent), and malpresentation (5 percent); more than one reason was allowed per woman.
Among the eligible women, 27 percent were studied. The reasons for nonparticipation were refusal by the woman (29 percent), refusal by the physician (10 percent), labor that was too active to permit consent (7 percent), failure of medical personnel to request participation (25 percent), and other miscellaneous reasons. Although only limited information was obtained on the eligible women who did not participate, the available data indicate that they were similar to the participants with respect to age and duration of pregnancy, but they were less likely to receive oxytocin during labor (33 percent vs. 39 percent).
A total of 928 women entered the study, 464 in each group. Three women were excluded from the analysis: one from each group for breech presentation not diagnosed at the time of randomization, and one from the amniotomy group who had obvious spontaneous rupture of the membranes before randomization. Among the remaining 925 women, 152 had cervical dilatation of less than 3 cm at randomization and 773 had cervical dilatation of 3 cm or more.
The women in the two treatment groups were similar with respect to base-line characteristics, both overall and within each cervical-dilatation group (Table 1). Among those in the amniotomy group, 96 percent had artificial rupture of the membranes in the first stage of labor, as compared with 51 percent in the conservative-management group. Among those in the conservative-management group who underwent amniotomy, the most frequently recorded indications were the failure of labor to progress (77 percent) and fetal distress (17 percent).
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3 cm initial cervical dilatation, early amniotomy was associated with a reduction of 125 minutes in this interval (260 minutes vs. 385 minutes, P<0.001). However, for women with less than 3 cm initial dilatation, the difference was smaller (442 minutes in the amniotomy group vs. 515 minutes in the conservative-management group, P = 0.06). The median duration of the second stage of labor was similar in the two groups (amniotomy, 68 minutes; conservative management, 70 minutes). Figure 2 shows the percentage of women in the two groups in whom labor continued as a function of the length of time after randomization.
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3 cm); among the 390 such women in the amniotomy group, 129 (33 percent) had dystocia, as compared with 183 of 383 (48 percent) in the conservative-management group (relative risk, 0.7; 95 percent confidence interval, 0.6 to 0.8). Among women with less than 3 cm initial cervical dilatation, the proportion of women with dystocia was 36 percent in the amniotomy group and 30 percent in the conservative-management group (relative risk, 1.2; 95 percent confidence interval, 0.8 to 1.9).
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The distribution of types of delivery was similar in the two groups (Table 3). Fifty-six women (12 percent) in the amniotomy group and 50 (11 percent) in the conservative-management group underwent cesarean section (relative risk, 1.1; 95 percent confidence interval, 0.8 to 1.6). In our analysis, the indication for cesarean section was that recorded on the women's hospital-discharge summary sheets. Dystocia was the sole indication for cesarean section for 26 women in the amniotomy group and 29 in the conservative-management group. Fetal distress was the sole indication for 12 women in the amniotomy group and 6 in the conservative-management group. Both fetal distress and dystocia were given as indications for cesarean section in the cases of 16 women in the amniotomy group and 9 in the conservative-management group. Cord prolapse occurred on one occasion in a woman in the conservative-management group, after the spontaneous rupture of the membranes. The rates of cesarean section according to the degree of initial cervical dilatation were as follows: for women with <3 cm initial cervical dilatation -- amniotomy, 14 of 72 women (19 percent), and conservative management, 10 of 80 women (12 percent); for women with 3 cm initial cervical dilatation -- amniotomy, 42 of 390 women (11 percent), and conservative management, 40 of 383 women (10 percent).
There were no statistically significant differences between the treatment groups in indicators of maternal morbidity (Table 4). The single maternal death occurred in a woman with undiagnosed Marfan syndrome whose aorta ruptured during labor.
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Discussion
It has been suggested that rates of cesarean section in North America could be reduced by more active management of labor according to protocols designed to prevent and treat functional disorders of labor1. In addition to early amniotomy, such protocols include selective admission to the labor ward, maternal ambulation, intensive social support, and the early recognition of delays in the progress of labor and consequent treatment with oxytocin. Impressively low rates of cesarean section have been reported in series in which active management has been used1,12. This study was designed to assess only one element of active management, early amniotomy, while attempting to minimize differences between the groups in other variables that could influence the progress of labor.
The main criterion by which progress in the first stage of labor is assessed is the pattern of cervical dilatation over time. There is disagreement about the degree of delay beyond which dystocia should be diagnosed and treatment instituted1,13,14,15. In this study, dystocia was defined according to the consensus guidelines established by an independent panel of experts7. These guidelines are very similar to those adopted by the World Health Organization's Working Group on the Partograph16. Delays in the progress of labor that occur when cervical dilatation is less than 5 cm include latent-phase disorders, which are considered by some to have less prognostic importance with respect to the method of delivery than delays occurring later in labor. In order to address this limitation of the study definition of dystocia, we performed a secondary analysis in which we defined dystocia as a delay of at least four hours after 5 cm of dilatation had been achieved during which the average rate of cervical dilatation was less than 0.5 cm per hour. The proportions of women with dystocia in the amniotomy and conservative-management groups according to this definition were 13 percent and 18 percent, respectively (relative risk, 0.7; 95 percent confidence interval, 0.5 to 1.0). The results of this second analysis indicate that a policy of early amniotomy reduces disorders of progress in the active phase of labor.
Despite the reduction in the frequency of dystocia for women with greater initial cervical dilatation ( 3 cm) who were assigned to routine early amniotomy, the frequency of cesarean section was not affected. Women in the conservative-management group were more likely than those in the amniotomy group to receive oxytocin. They also frequently underwent amniotomy for delay in the progression of labor. Thus, the medical treatment of established dystocia appears to have canceled any excess risk of cesarean section in the conservative-management group.
A recently published randomized study17 compared the effect of two elements of active management (early amniotomy and early adminstration of oxytocin) with that of routine care on the risk of cesarean section. After adjustment for several confounding variables, the analysis suggested that active management may reduce the risk of cesarean section. These results contrast not only with our findings but also with those of two previous randomized studies that assessed these combined elements of active management18,19.
In our study, cesarean sections for which fetal distress was an indication were more frequent in the amniotomy group than in the conservative-management group. This finding raised the possibility of an effect of early amniotomy on fetal-heart-rate patterns. In contrast to some previous reports,3,20 however, we found no evidence of an effect of amniotomy on the incidence of disorders of the fetal heart rate. Despite the absence of objective evidence of an effect on fetal heart rates, it is possible that factors related to membrane status (such as earlier knowledge of the presence of meconium or earlier use of internal monitoring) could have influenced the physician's interpretation of the tracings in the clinical setting.
The two groups were similar with respect to indicators of the condition of the babies delivered. Although there was a trend to fewer abnormal one-minute and five-minute Apgar scores in the amniotomy group, the frequency of abnormal cord-blood pH values was similar in the two groups. In all four randomized clinical trials (including the present study) from which categorical data on five-minute Apgar scores are available, the frequency of abnormal scores has been lower in the amniotomy group2,6,19. The reason for this difference is not known.
In summary, for nulliparous women in spontaneous labor, routine amniotomy before dilatation of the cervix has reached 3 cm has no benefit. When the degree of cervical dilatation is 3 cm or greater, amniotomy is associated with a reduction in both the duration of labor and the frequency of dystocia. Other possible benefits of a policy of routine amniotomy that require further assessment include reductions in the frequency of administration of oxytocin and in the risk of abnormal five-minute Apgar scores in the newborn infants. There is no evidence that early amniotomy reduces the frequency of cesarean section.
Supported by an operating grant (MA-10556) and a nominal award (DG-401 [to Dr. Fraser]) from the Medical Research Council of Canada.
Source Information
From the Departments of Obstetrics and Gynecology (W.D.F., J.-M.M.) and Social and Preventive Medicine (S.M., A.C.), Laval University, Quebec, Canada. The members of the study group are listed in the Appendix.
Address reprint requests to Dr. Fraser at DO-705-Perinatologie, Hopital St.-Francois d'Assise, 10 rue de l'Espinay, Quebec, QC G1L 3L5, Canada.
References
The following investigators and nurses were members of the Canadian Early Amniotomy Study Group: B.A. Armson and K. Phalen-Kelly (Grace Maternity Hospital, Halifax, N.S.); J.P. Verrault, G. Paradis, and G. Poulin (St. Sacrement Hospital, Quebec City, Que.); L. Moreau (Hopital St.-Francois d'Assise, Quebec City, Que.); N. Okun, C. Nimrod, and M. Villeneuve (Ottawa General Hospital, Ottawa, Ont.); A.K. Joshi and C. Nault (St. Mary's Hospital, Montreal); H. Cohen and J. Weston (Women's College Hospital, Toronto); T. Doran and A. Jordan (Toronto General Hospital, Toronto); P. Bernstein, J. Carroll, and C. Pierce (Mt. Sinai Hospital, Toronto); L. Bayer and A. Kang (St. Michael's Hospital, Toronto); S. Bottoms and G. Norman (Hutzel Hospital, Detroit); and F. Galerneau and P. Jansen (Grace Hospital, Vancouver, B.C.).
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Related Letters:
Amniotomy in Labor
Teoh T.G., Robson M.S., Boylan P.C., Fraser W., Marcoux S., Moutquin J.-M.
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Full Text
N Engl J Med 1993;
329:886-887, Sep 16, 1993.
Correspondence
This article has been cited by other articles:
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