Uterine rupture remains one of the most catastrophic obstetric emergencies.1,2 Interdelivery and interpregnancy intervals have been associated with the likelihood of uterine rupture and now represent risk factors that should be considered in the management of women contemplating a vaginal birth after cesarean (VBAC).3–6 In 2001, Shipp et al reported that an interdelivery interval shorter than 18 months was associated with a higher risk of uterine rupture than an interval longer than 18 months.4 In 2002, we found that an interdelivery interval between 12 and 24 months was also associated with a higher risk of uterine rupture than an interval of 24 months or more,5 but we did not investigate the risk of uterine rupture for any other cutoff between 12 and 24 months. In 2004, the American College of Obstetricians and Gynecologists stated that women who attempt VBAC who have interdelivery intervals less than 24 months have a twofold to threefold higher risk of uterine rupture when compared with women who attempt VBAC more than 24 months after their last delivery.7 The same year, the Society of Obstetricians and Gynaecologists of Canada suggested that an interdelivery interval less than 18–24 months should be viewed as a risk factor for uterine rupture, leaving clinicians in a “gray zone” when counseling women with an interdelivery interval between 18 and 24 months.8 We attempt to correct this uncomfortable situation and reviewed our more recent data for this patient subgroup that constitutes a significant proportion of women. The objective of our study was to estimate the association between interdelivery interval and uterine rupture in women with a previous cesarean delivery.
MATERIALS AND METHODS
Our retrospective cohort study comprised women with a single previous cesarean delivery who were admitted to Sainte-Justine Hospital between January 1987 and December 2004 to undergo a trial of labor. Part of this database was investigated in the past and reported in several publications.5,9–13 Medical records were reviewed by two independent observers who collected demographic data, medical and obstetric history, complications, and outcomes of the current pregnancy as well as birth weight. The inclusion criteria were singleton pregnancies at 37 or more weeks of gestation at delivery in patients with only one previous low, transverse cesarean delivery. The decision to exclude women undergoing a trial of labor before term was predetermined and based on several factors: 1) they were at very low risk of uterine rupture; 2) it provided a better estimate of the risk for women at term; and 3) we did not have to adjust for length of the current pregnancy. Women with a prior, classical, J-shaped, T-inverted incision or prior transmural myomectomy were excluded, as were women with vaginal delivery or mid-trimester fetal delivery (including spontaneous abortion or voluntary abortion after 14 weeks of gestation) between the previous cesarean delivery and the current pregnancy. The study population was divided into three groups according to the interdelivery interval, defined as the time between the date of the previous cesarean delivery and the date of the trial of labor: 24 months or longer (control group), 18 to 24 months, and fewer than 18 months.
The rate of symptomatic uterine rupture—our primary outcome, defined as complete separation of the uterine scar, resulting in communication between the uterine and peritoneal cavities and requiring emergency cesarean delivery or postpartum laparotomy—was compared across groups. Multivariable logistic regression analyses with and without stepwise selection of covariates were performed to control for potential confounding factors. The following covariates were included in the model: prior uterine closure, interdelivery interval (as categorical variable), labor induction, use of prostaglandins, prior vaginal delivery, maternal age older than 35 years, gestational age at delivery 41 weeks or later, and birth weight more than 4,000 g. The associations were reported as odd ratios with 95% confidence intervals. Medians were compared using Kruskal-Wallis test and proportions were compared using Pearson's χ2 test or the Fisher exact test when appropriate. Statistical analyses were conducted with SPSS 16.0 (Chicago, IL) and P less than .05 was designated to indicate statistical significance. Approval from the Ethics and Scientific Committee of Sainte-Justine Hospital was obtained before the beginning of the study.
Between January 1987 and December 2004, 1,787 women who met the inclusion and exclusion criteria underwent a trial of labor at term. The date of the previous cesarean delivery and, therefore, the interdelivery interval were not available for 19 (1%) of them, so they were excluded. Of the 1,768 women analyzed, 1,323 (74.8%) had an interdelivery interval of 24 months or longer, 257 (14.5%) had an interval between 18 and 24 months, and 188 (10.6%) had an interval of less than 18 months. Demographic characteristics are reported in Table 1. The rate of successful VBAC (70%, 74%, and 73%, respectively; P=.28) was not different across the three groups, but women with short interdelivery intervals were more likely to be younger and to have had a single-layer closure of their hysterotomy at the time of previous cesarean delivery. The rates of uterine rupture were 1.3% (17 of 1,323), 1.9% (5 of 257), and 4.8% (9 of 188), respectively (P=.003). However, the rate of 5-minunte Apgar score less than 7 was not different among the groups (2.5%, 2.4%, 1.7%, respectively; P=.81). After adjustment for confounding variables, an interdelivery interval less than 18 months was associated with a significant increase of uterine rupture (odds ratio [OR], 3.0; 95% confidence interval [CI], 1.3–7.2), whereas an interdelivery interval between 18 and 24 months was not (OR, 1.1; 95% CI, 0.4–3.2) (Table 2). In a stepwise logistic regression analysis, only two covariates remained associated with a significant increase of uterine rupture and in the model: a previous single-layer closure (OR, 7.5; 95% CI, 3.2–17.6) and an interdelivery interval less than 18 months (OR, 2.8; 95% CI, 1.2–6.6). Again, an interdelivery interval between 18 and 23 months was not associated with a significantly greater risk of uterine rupture than an interdelivery interval of 24 months or longer (OR, 1.2; 95% CI, 0.4–3.2). The rate of uterine rupture for an interdelivery interval between 15 and 18 months (6 of 83, 7.2%) was not statistically different than the rate for an interdelivery interval of 15 months or less (3 of 105, 2.9%; Fisher exact test: P=.19), but the power was limited by the small number of women in each group.
As reported in 2002 from a portion of the same database, we observed a positive association between interdelivery interval and uterine rupture. More specifically, the risk of uterine rupture was significantly higher in women with an interdelivery interval less than 18 months than in women with an interval of 24 months or longer, but not in women with an interval between 18 and 23 months, inclusively.
Our findings are also in complete agreement with those of Shipp et al, who discerned a high risk of uterine rupture in women with an interdelivery interval less than 18 months, and with the studies of Esposito et al and Stamilio et al, who noted a high risk of uterine scar defect or uterine rupture in women with an interpregnancy interval less than 6 months, which could be compared with an approximate interdelivery interval of 15 months for women with a second pregnancy at term.3,4,6 In a very large (13,331 women), multicenter, retrospective, cohort study, Stamilio et al reported an adjusted OR for uterine rupture of 3.05 (95% CI, 1.36–6.87) in women with an interpregnancy interval less than 6 months, of 1.18 (95% CI, 0.60–2.33) in women with an interpregnancy interval between 6 and 11 months, and of 1.00 (95% CI, 0.56–1.79) for women with an interpregnancy interval between 12 and 17 months, compared with women with an interpregnancy interval between 18 and 59 months.6 Therefore, their observations fully support ours. Unfortunately, this study was limited because the interpregnancy interval was calculated by using delivery immediately before the study pregnancy, regardless of the delivery mode of that prior delivery, and also by approximating the interpregnancy interval (the cohort database included only the year of the prior delivery and set the prior delivery date as January 1 of the delivery year for each patient). We found three other studies that did not observe an association between interdelivery intervals and uterine rupture.14–16 Huang et al found that an interdelivery interval less than 19 months was associated with a lower rate of successful VBAC but not with uterine rupture.14 This study was limited by its size and the number of uterine ruptures (only three) reported. Ridgeway et al found no difference in the mean interdelivery interval between women who had a uterine rupture and a control groups without uterine rupture.15 However, they did not define the interdelivery interval, and women with multiple cesarean deliveries and prior VBAC were not excluded. Finally, in the study of Grobman et al, interdelivery interval was not a significant factor that remained in their final logistic regression model using a stepwise variable selection, and the crude rate of uterine rupture for short interdelivery intervals was not reported.16 Therefore, we cannot find any study that suggests an increased risk of uterine rupture for women with an interdelivery interval between 18 and 24 months.
Our study remains limited by its retrospective nature and by its inclusion of women who underwent a trial of labor after cesarean delivery over a long observation period from the late 1980s and early 1990s, when there were no guidelines regarding some risk factors (unknown at that time), including labor induction with unfavorable cervix, use of prostaglandins, prolonged dystocia, and single-layer closure. On the other hand, prostaglandin treatment was rare (less than 1%) in our population, and we undertook multivariate logistic regression analysis to adjust for these potential confounding factors. Although the crude rate of uterine rupture for each subgroup was probably not applicable to actual practice where several risk factors were taken into account and the management of labor was most likely quite different, we believe that the association between interdelivery interval and uterine rupture is valid. Finally, the women in our cohort did not undergo lower uterine segment measurement, and therefore we cannot appreciate the potential impact of such information. However, in a recent publication, we found that an interdelivery interval less than 18 months was an independent and an important factor associated with uterine scar defect, after adjustment for a uterine scar thickness less than 2.3 mm and a previous single-layer closure of the uterus.17 Further studies are required to better estimate the role of this tool for women with a short interdelivery interval.
In conclusion, we recommend that women with an interdelivery interval between 18 and 24 months, who represent approximately 15% of women seeking VBAC, should not be precluded from a trial of labor based on this unique factor. On the other hand, we believe that women with an interdelivery interval less than 18 months should be informed of the high risk of uterine rupture during a trial of labor.
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© 2010 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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