Volume 200, Issue 5 , Pages e12-e15, May 2009
Is cerclage height associated with the incidence of preterm birth in women with an ultrasound-indicated cerclage?
Article Outline
Objective
Our aim was to determine whether there was a cerclage height threshold associated with spontaneous preterm birth in patients with an ultrasound-indicated cerclage.
Study Design
We performed a retrospective cohort study of women with an ultrasound-indicated cerclage. Functional cervical length and the cerclage height (distance from cerclage to the external cervical os) were obtained. Our cohort was grouped into thirds, based on cerclage height percentile. Our primary outcome was spontaneous preterm birth less than 35 weeks.
Results
There were 20 women in group 1 (< 18 mm), 25 in group 2 (13-17 mm), and 25 in group 3 (≥ 18 mm). Women with cerclage height 18 mm or greater had a lower incidence of spontaneous preterm birth less than 35 weeks (4%) when compared with those with a cerclage height less than 18 mm (33%) (relative risk, 0.69; 95% confidence interval, 0.55-0.86).
Conclusion
Cerclage height of 18 mm or greater is associated with a reduction in spontaneous preterm birth for women with an ultrasound-indicated cerclage.
Key words: cerclage, cervical length, preterm birth
The shortened cervix as determined by transvaginal ultrasound is a well-established predictor of preterm birth.1 Cervical cerclage is 1 intervention that is used for women found to have a shortened cervix in the midtrimester, referred to as the ultrasound-indicated cerclage. The efficacy of ultrasound-indicated cerclage has been evaluated against no cerclage in randomized controlled trials, but the results have been mixed.2, 3, 4, 5 From a recent metaanalysis of patient level data, it was reported that cervical cerclage reduced the risk of preterm birth for a select population, those with a singleton pregnancy, history of preterm birth, and a shortened cervix in the midtrimester.6
In addition to the patient population, other factors such as cerclage position may contribute to the variability observed in cerclage efficacy. With transvaginal ultrasound the distance from cerclage to the external os, termed cerclage height, can be reliably visualized and measured.7, 8, 9, 10 Prior studies have demonstrated variability in cerclage height between patients,9, 10 which may affect cerclage efficacy. We hypothesized that the greater the cerclage height placement, the more likely the cerclage would be efficacious. Our objective was to determine, in patients with an ultrasound-indicated cerclage, whether there was a cerclage height threshold associated with spontaneous preterm birth.
Materials and Methods
We performed a retrospective cohort study of women who received an ultrasound-indicated cerclage. We defined ultrasound-indicated cerclage as a cerclage placed for a short cervical length (< 25 mm), detected by transvaginal ultrasound between 14 and 236/7 weeks in asymptomatic women. Our data sources were the Thomas Jefferson University Prematurity Database, which included women from 1995 to 2007, and the Main Line Hospital cerclage database, which included women from 2003 to 2007. History-indicated cerclage, transabdominal cerclage, multiple gestations, indicated preterm birth, and pregnancies affected by a major fetal anomaly were not included in the analysis to keep our study population uniform.
Ultrasound measurements prior to and after cerclage placement (if < 24 weeks) were used for data analysis. All transvaginal ultrasound measurements were performed by experienced sonographers using standard technique. The patients' bladder was emptied prior to visualization of the cervix. Only the minimum pressure necessary was used to obtain a clear image of the cervical canal in the midsagittal plane. The first ultrasound obtained after placement of the cerclage was performed within 2 weeks.
The following cervical ultrasound measurements were considered in the analysis: functional cervical length (closed portion of the endocervical canal) and the cerclage height (the distance from cerclage to the external cervical os) (Figure 1). At least 3 measurements each were obtained for the functional cervical length and cerclage height, and the shortest measurement was used. The managing obstetricians were not blinded to the measurement results. All data analysis was based on the shortest measurements for each of the obtained measurements between 14 and 236/7 weeks.
FIGURE 1.
Schematic representation of cervical length and cerclage height ultrasound measurements
The opaque circles represent the cerclage.
Scheib. Cerclage height and the incidence of preterm birth in ultrasound-indicated cerclage. Am J Obstet Gynecol 2009.
We analyzed our cohort by grouping patients into thirds, based on cerclage height percentile prior to cerclage. Our primary outcome was spontaneous preterm birth less than 35 weeks and weeks gained after placement of cerclage. The χ2 or Fisher's exact tests were performed for categorical variables. Analysis of variance was performed for continuous variables. Type I error was set at 0.05 (2 sided). Logistic regression was performed to assess for confounders. Variables that were associated with cerclage height (P < .2) were considered as potential confounders and included in the multivariable logistic regression model. All statistical analysis was performed using SPSS software (version 16; SPSS Inc, Chicago IL).
Results
Seventy women with ultrasound-indicated cerclage, singleton gestation, and complete ultrasound measurements were identified. There were 20 women in group 1 (< 13 mm), 25 in group 2 (13-17 mm), and 25 in group 3 (≥ 18 mm). The baseline characteristics were similar between groups, with the exception of prior preterm birth and cervical length at placement (Table). Sixty-three of the 70 cerclages placed were McDonald cerclages. It was unclear the cerclage type of the remaining 7 cerclages. Most of the cerclages were placed by residents as the primary surgeon under the supervision of a maternal fetal medicine attending. The majority of the cerclages used Mersilene tape.
TABLE. Baseline characteristics by cerclage height group
| Total | Group 1 (< 13mm) | Group 2 (13-17 mm) | Group 3 (≥ 18 mm) | P value | |
|---|---|---|---|---|---|
| n | 70 | 20 | 25 | 25 | |
| Age (SD)a | 27 (5) | 27(4) | 28(6) | 28(4) | .869b |
| Parity (SD)a | 1.1(1.2) | 1.0(1.0) | 0.9(1.1) | 1.4(1.5) | .417b |
| African American race (%) | 39(58) | 13(65) | 13(52) | 15(60) | .668c |
| Prior preterm birth < 35 weeks (%) | 19(27) | 9(45) | 7(28) | 3(12) | .047d |
| Prior cervical conization (%) | 12(18) | 3(16) | 6(25) | 3(12) | .488d |
| Mullerian anomaly (%) | 3(4) | 0 | 1(4) | 2(8) | .455d |
| Gestational age at cerclage in weeks (SD)a | 19(1.9) | 18(2) | 19(1) | 19(1) | .120b |
| Functional cervical length prior to cerclage (mm) (SD)a | 20(5) | 17(7) | 19(4) | 23(3) | .002b |
aMean; |
bAnalysis of variance; |
cχ2 test; |
dFisher's exact test. |
The incidence of spontaneous preterm delivery was 30% (6/20) in group 1, 36% (9/25) in group 2, and 4% (1/25) in group 3 (Figure 2). A significant difference was found in the incidence of spontaneous preterm birth according to cerclage height groups (P = .018). There was no difference in the incidence of spontaneous preterm delivery between groups 1 and 2 (P = .671), although there was a difference in the incidence of preterm delivery between both groups 1 and 3 and 2 and 3 (P = .034 and P = .011, respectively). Because there was no significant difference between groups 1 and 2, these 2 groups were then combined and compared with group 3 for further analysis (P = .0035). Women with cerclage height of 18 mm or greater had a lower incidence of spontaneous preterm birth less than 35 weeks, 1 of 25 (4%), when compared with those with a cerclage height less than 18mm 15 of 45 (33%) (relative risk, 0.69; 95% confidence interval, 0.55-0.86). Post hoc analysis found the observed power to be 1.
FIGURE 2.
Incidence of preterm birth < 35 weeks according to cerclage height
Bar graph representation of the incidence of preterm birth < 35 weeks according to cerclage height group.
Scheib. Cerclage height and the incidence of preterm birth in ultrasound-indicated cerclage. Am J Obstet Gynecol 2009.
An analysis of outcomes according to weeks gained after cerclage placement was looked at to control for the gestational age at cerclage placement. Women with a cerclage height of 18 mm or greater had significantly greater amount of time gained (17.2 weeks) when compared with those with a cerclage height of less than 18 mm (15.2 weeks; P = .047).
The potential confounders, prior preterm birth, gestational age at cerclage, and functional cervical length prior to cerclage, were included in the logistic regression analysis along with cerclage height of 18 mm or greater. In our model the associated reduction in preterm birth less than 35 weeks remained only for those with a cerclage height of 18 mm or greater (adjusted odds ratio, 0.10; 95% confidence interval, 0.01-0.94).
Comment
Placing an ultrasound-indicated cerclage at a cerclage height (distance from the external os) of 18 mm or greater was associated with a lower incidence of spontaneous preterm birth compared with placing the cerclage closer to the external os. Importantly, this held true even when we controlled for functional cervical length, which, if short, could make it more difficult to place the cerclage close to the internal os.
Whereas several randomized studies have assessed the efficacy of ultrasound-indicated cerclage,2, 3, 4, 5 there is paucity of data on technical aspects of this procedure, which is something over which the surgeon has some control. Few studies have evaluated whether a successful cerclage depends on suture placement as close as possible to the internal cervical os, which can be represented by cerclage height. As originally described in 1957 by McDonald,11 the cerclage was placed “as high as possible to approximate to the level of the internal os.” After more than 50 years, our findings support this practice in women with ultrasound-indicated cerclage.
Similar to our study, Rust et al10 investigated ultrasound-indicated cerclage position and the incidence of preterm birth in 74 women. Their linear regression analysis demonstrated wide variability but no association between cerclage height and preterm birth.10 Because there did not appear to be a linear association between cerclage height and preterm birth based on data from the Rust article, we postulated that there may be a cutoff value for cerclage height that may be associated with preterm birth. This was supported by our finding that women with a cerclage height of 18 mm or greater had a much lower incidence of preterm birth.
Cerclage height has been evaluated also in women with other indications for cerclage. In 29 women, physical examination indicated (also called emergency) cerclage, cerclage height was found to positively correlate with gestational age at delivery, but the association did not persist in the multivariable analysis.9 Our hypothesis of why cerclage height is associated with lower rates of preterm birth is that the cerclage is closer to the internal os and thus provides reinforcement. When comparing outcomes between women with similar prior failed transvaginal cerclage who had either a transabdominal or a transvaginal cerclage, the incidence of preterm birth was lower in women with a transabdominal cerclage, which is placed directly at the internal cervical os.12
Other ultrasound measures of cerclage position aside from cerclage height have been studied as predictors of preterm delivery. Guzman et al9 found that the distance from the cerclage to the internal os (upper cervical length) correlated minimally (r = 0.31) with gestation age at delivery, but this association was not significant (P = .10). An upper cervical length of less than 10 mm was, however, associated with delivery at less than 36 weeks. This cutoff value was predictive only if measured in the first 48 hours after cerclage placement because the upper cervical length was found to shorten to less than 10 mm before 28 weeks in all patients.9 Funneling to the cerclage has also been found to be predictive of preterm delivery, but like shortening of upper cerclage height, funneling may develop at any time on subsequent ultrasounds and would require serial scans after cerclage placement.13
Conclusions from our study need to be considered in light of the relatively small sample size. Although equally as large as any similar study reported in the literature, the 95% confidence interval for our primary outcome is wide (0.01- 0.94) as a result of the sample size. We elected to narrow our patient population by excluding patients with a history-indicated cerclage or a physical examination–indicated cerclage to keep our study population homogenous. Other variables such as inflammation could have affected our results.
Our study suggests that a cerclage height of 18 mm or greater was achieved in about a third of women who have an ultrasound-indicated cerclage and that these patients appear have a much lower risk of preterm delivery compared with patients with a shorter cerclage height. We suggest attempting to place the cerclage as close as possible to the internal os. In the greater than 13 years since we began recording outcomes with ultrasound-indicated cerclage, with most placed by residents as primary surgeons with supervision of a maternal fetal-medicine attending physician, no bladder injuries or fistulas occurred. Transvaginal ultrasound measurement of cerclage height may provide the clinician with a useful tool to evaluate the effectiveness of the cerclage placement and provide more accurate counseling regarding the risk of preterm delivery following an ultrasound-indicated cerclage.
References
- The length of the cervix and the risk of spontaneous premature delivery (National Institute of Child Health and Human Development Maternal Fetal Medicine Unit Network). N Engl J Med. 1996;334:567–572
- . Cerclage for prevention of preterm birth in women with a short cervix found on transvaginal ultrasound examination: a randomized trial. Am J Obstet Gynecol. 2004;191:1311–1317
- . Final results of the Cervical Incompetence Prevention Randomized Cerclage Trial (CIPRACT): therapeutic cerclage with bed rest versus bed rest alone. Am J Obstet Gynecol. 2001;185:1106–1112
- Cervical cerclage for prevention of preterm delivery in women with short cervix: randomised controlled trial. Lancet. 2004;363:1849–1853
- . Revisiting the short cervix detected by transvaginal ultrasound in the second trimester: why cerclage therapy may not help. Am J Obstet Gynecol. 2001;185:1098–1105
- . Cerclage for short cervix on ultrasonography: meta-analysis of trials using individual patient-level data. Obstet Gynecol. 2005;106:181–189
- . Vaginal ultrasound and cervical cerclage: a prospective study. Ultrasound Obstet Gynecol. 1992;2:410–416
- . Prediction of cervical cerclage outcome by endovaginal ultrasonography. Am J Obstet Gynecol. 1994;171:1102–1106
- . The significance of transvaginal ultrasonographic evaluation of the cervix in women treated with emergency cerclage. Am J Obstet Gynecol. 1996;175:471–476
- . Does cerclage location influence perinatal outcome?. Am J Obstet Gynecol. 2003;189:1688–1691
- . Suture of the cervix for inevitable miscarriage. J Obstet Gynaecol Br Emp. 1957;64:346–350
- . Patients with a prior failed transvaginal cerclage: a comparison of obstetric outcomes with either transabdominal or transvaginal cerclage. Am J Obstet Gynecol. 2000;183:836–839
- . Funneling to the stitch: an informative ultrasonographic finding after cervical cerclage. Ultrasound Obstet Gynecol. 2002;20:252–255
Reprints not available from the authors.
PII: S0002-9378(08)01076-4
doi:10.1016/j.ajog.2008.09.021
© 2009 Mosby, Inc. All rights reserved.
Volume 200, Issue 5 , Pages e12-e15, May 2009
