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Transvaginal Ultrasonography of the Cervix to Predict Preterm Birth in Women With Uterine Anomalies

From the Divisions of Maternal-Fetal Medicine, Departments of Obstetrics and Gynecology, ¹Jefferson Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania, and ²Pennsylvania Hospital, Philadelphia, Pennsylvania.

	ABSTRACT

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Objective: Women with uterine anomalies have higher rates of preterm birth, but the reason for this has not been elucidated. Transvaginal ultrasound examination has been shown to be an accurate test for the prediction of preterm birth but has not been studied specifically in this population.

Methods: Pregnant women with uterine anomalies were followed prospectively with transvaginal ultrasound examination of the cervix, performed between 14 and 23 6/7 weeks of gestation. A short cervical length was defined as less than 25 mm of cervical length. The primary outcome was spontaneous preterm birth, defined as birth at less than 35 weeks.

Results: Of the 64 pregnancies available for analysis, there were 28 with a bicornuate uterus, 13 with a septate uterus, 11 with a uterine didelphys, and 12 with a unicornuate uterus. The overall incidence of spontaneous preterm birth at less than 35 weeks was 11%. Of the 10 (16%) women with a short cervical length, 5 (50%) had spontaneous preterm birth. Of the 54 women without a short cervical length, only 2 (4%) had a spontaneous preterm birth. The sensitivity, specificity, and positive and negative predictive values of a short cervical length for spontaneous preterm birth were 71%, 91%, 50%, and 96%, respectively (relative risk 13.5, 95% confidence interval 3.49–54.74). Of the 7 women with both short cervical length and preterm birth, all uterine subtypes were represented except septate uterus.

Conclusion: A short cervical length on transvaginal ultrasonography in women with uterine anomalies has a 13-fold risk for preterm birth. Unicornuate uterus had the highest rate of cervical shortening and preterm delivery.

Level of Evidence: II-2

Uterine anomalies are a known risk factor for preterm birth. Congenital uterine malformations are more common than generally recognized. A recent review showed that uterine anomalies were present in 1 in 594 fertile women (0.17%), 1 in 29 infertile women (3.5%), and 1 in 201 women from the general public (0.5%).⁶ Congenital uterine anomalies have been associated with infertility, recurrent early pregnancy loss, preterm birth, malpresentation, and cesarean delivery. The objective of the current study was to estimate the predictive value of a short cervical length as measured by transvaginal ultrasonography for spontaneous preterm birth in women with uterine anomalies.

	MATERIALS AND METHODS

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Pregnant women with a history of uterine anomalies (bicornuate uterus, unicornuate uterus, septate uterus, or uterus didelphys) were followed prospectively from 14 through 23 6/7 weeks gestation with transvaginal ultrasound examination of the cervix approximately every 2 weeks. Uterine anomalies were based on the classification system of Buttram and Gibbons.⁷ Müllerian fusion defects were diagnosed by 3D sonography, hysterosalpingography, or hysteroscopy. Exclusion criteria were history-indicated cerclage, multiple gestation, medically indicated preterm birth, currently undelivered, and lack of transvaginal ultrasound information. In this study, all women were seen between 1995 and 2004 at Thomas Jefferson University Hospital or Pennsylvania Hospital. The institutional review board at Thomas Jefferson University Hospital approved this study.

Short cervical length was defined as a cervical length less than 25 mm. All cervical length measurements were performed by experienced sonographers using standard technique. Following visualization of the cervix, only the minimum pressure necessary was used to obtain a clear image of the cervical canal. Multiple cervical measurements were obtained. Managing obstetricians were not blinded to cervical length results. All data analysis was based on the shortest cervical length obtained between 14 and 23 6/7 weeks.

The primary outcome was incidence of spontaneous preterm birth before 35 weeks of gestation. A sample size calculation showed that, assuming a background risk of preterm birth at less than 35 weeks of 12%, a 40% incidence of short cervix, a relative risk of 4 for preterm birth with a short cervical length, an alpha of 0.05 and power of 80%, a total of approximately 70 women with uterine anomalies would be needed. Subanalyses, according to the 4 types of uterine anomalies (bicornuate, unicornuate, septate, and didelphys), were planned, with an awareness of type II error limitations of these analyses.

Statistical analysis was performed with the SPSS 13.0 statistical package (SPSS Inc, Chicago, IL). Fisher exact test and ² were used for calculating statistics where appropriate.

To determine whether any prior study had investigated this topic, a MEDLINE and PubMed literature search was performed using the key words "uterine anomaly," "müllerian anomaly," "transvaginal ultrasound," "preterm birth," "bicornuate," "unicornuate," "septate uterus," and "didelphys uterus." No articles were found that specifically addressed the use of transvaginal ultrasonography in women with müllerian anomalies for the prediction of preterm birth. No other study investigating our topic was found in the literature as of April 2005.

	RESULTS

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Ninety-four women were identified. Of these, 30 women were excluded, leaving 64 for data analysis. Criteria for exclusion were history-indicated cerclage (n = 7), ultrasonography-indicated cerclage (n = 4), multiple gestation (n = 9), exact müllerian anomaly unknown (n = 3), medically indicated preterm birth at less than 35 weeks (n = 2), not delivered yet (n = 1), and no transvaginal ultrasound information (n = 4). The overall incidence of short cervical length was 16%, and the overall incidence of preterm birth at less than 35 weeks was 11%. There was no difference in demographic characteristics between the 2 groups, ie, the group with cervical length of less than 25 mm and that with cervical length of 25 mm or more (Table 1). Of the 10 women with a short cervical length, 5 (50%) women had a spontaneous preterm birth. Of the 54 women with a normal cervical length, only 2 (4%) women had a spontaneous preterm birth (Fig. 1). The sensitivity, specificity, and positive and negative predictive values of a short cervical length for spontaneous preterm birth in women with uterine anomalies were 71% (95% confidence interval [CI] 39–91%), 91% (95% CI 87–94%), 50% (95% CI 27–64%), and 96% (95% CI 92–99%), respectively, with a relative risk of 13.5 (95% CI 3.49–54.74). Table 2 demonstrates the characteristics of all of the different uterine anomalies. The most frequent subtype was the bicornuate uterus, and the least frequent subtype was the didelphys uterus. Overall, the unicornuate uterus had the highest rate of cervical shortening and the highest rate of spontaneous preterm birth when the cervix shortened. The septate uterus had the lowest rate of preterm birth. Both of the women who did not develop a short cervix, yet had a preterm birth, were women with uterine didelphys.

View larger version (21K): [in this window] [in a new window]   Fig. 1. Flow diagram segregated by cervical length and preterm birth. Airoldi. Prediction of Preterm Birth in Uterine Anomalies. Obstet Gynecol 2005.

	DISCUSSION

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In this study, a short cervical length as measured by transvaginal ultrasonography was shown to be a good predictor of spontaneous preterm birth in a highly selected cohort of women, ie, those with uterine anomalies. These women, previously shown to be at high risk for preterm birth, had never been followed by this screening test, transvaginal ultrasonography of the cervix. Our data confirms that, in women with congenital uterine anomalies, a short cervical length has a 13-fold risk of preterm birth. Although transvaginal cervical ultrasonography had been described to be predictive of preterm birth in other populations,² this is the first such study in women with uterine anomalies.

In a prior study,⁸ transabdominal ultrasonography of the cervix was used to screen 30 women with a prior preterm birth and uterine anomalies. Eighty percent of these women had cervical changes on transabdominal ultrasonography and received a cerclage. The current study shows that the risk of developing a short cervical length, as measured by transvaginal ultrasonography, to be increased (16 %), but lower than previously described by Ayers et al.⁸ Note that the 2 groups of women in the 2 studies differ in that all patients in the study by Ayers had a prior preterm birth, which was not the case in our study.

Prior literature has shown wide variations in the data indicating which uterine anomaly has the highest rate of preterm birth (didelphys,⁹ septate¹⁰). The current study showed that the unicornuate uterus had the highest risk of preterm birth when the cervix shortened, with transvaginal ultrasonography having a high predictive accuracy. On the contrary, in the didelphys group, there were 2 women with no cervical changes who went on to deliver preterm. Because the entire didelphys group was only 11 women, no firm conclusions can be made, but only hypotheses for further study.

There are shortcomings in this study. First, the obstetricians providing the care were not blinded as to the results of the transvaginal ultrasonography. The study originally had 94 women with uterine anomalies, but this number was lowered to 64 because we excluded 30 patients to keep our study group more homogeneous.

The current study showed that transvaginal ultrasonography of cervical length is a good screening test for preterm birth in women with uterine anomalies. Because our study is the first to assess the predictive accuracy of this screening test in this population, firm recommendations cannot be made. Our suggestion, if the clinician and patient decide to screen with transvaginal ultrasonography, is that, if the cervix stays greater than 2.5 cm, no additional treatment is necessary because the risk of preterm birth is extremely low (4% in our study). The high negative predictive value will help to reassure these women, and the clinician need not perform unnecessary interventions (eg, bed rest or cerclage). If the cervix shortens to less than 2.5 cm, the patient is counseled regarding her increased risk of preterm birth (50% in our study). Unfortunately, until an intervention is proven to prevent preterm birth in these women, this screening test should not be uniformly recommended. The information provided here can help in the counseling of women with uterine anomalies who are pregnant or are planning a pregnancy.

	Footnotes

 
Corresponding author: James Airoldi, MD, Division of Maternal–Fetal Medicine, Department of Obstetrics and Gynecology, Jefferson Medical College of Thomas Jefferson University, 834 Chestnut Street, Suite 400, Philadelphia, PA 19107; e-mail: james.airoldi{at}mail.tju.edu.

doi:10.1097/01.AOG.0000173987.59595.e2

	REFERENCES

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5. Berghella V, Daly SF, Tolosa JE, DiVito MM, Chalmers R, Garg N, et al. Prediction of preterm delivery with transvaginal ultrasonography of the cervix in patients with high risk pregnancies: Does cerclage prevent prematurity? Am J Obstet Gynecol 1999;181:809–15.[Medline]

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