Advertisement

Can transabdominal ultrasound be used as a screening test for short cervical length?

Published:December 14, 2012DOI:https://doi.org/10.1016/j.ajog.2012.12.021

      Objective

      Universal transvaginal cervical length screening can be associated with a significant logistical burden. We hypothesized that there is a threshold cervical length measured by transabdominal ultrasound above which risk for short transvaginal cervical length is extremely low.

      Study Design

      This prospective cohort study evaluated a consecutive series of women offered universal transvaginal cervical length screening during anatomy ultrasound. Transabdominal measurement of the cervix—obtained before and after voiding for each patient—was performed before transvaginal ultrasound. The study was powered to detect a transabdominal cervical length cutoff with 95% sensitivity (95% confidence interval, 90–99%) for transvaginal cervical length of ≤25 mm.

      Results

      One thousand two hundred seventeen women were included in the analysis. Prevoid transabdominal cervical length ≤36 mm detects 96% of transvaginal cervical lengths ≤25 mm with 39% specificity. A prevoid transabdominal cervical length ≤35 mm detects 100% of transvaginal cervical lengths ≤20 mm with 41% specificity. Transabdominal images of the cervix could not be obtained in 6.2% of women prevoid and 17.9% of women postvoid.

      Conclusion

      Transabdominal cervical length screening successfully identifies women at very low risk for short transvaginal cervical length. Transabdominal screening may significantly reduce the burden of universal cervical length screening by allowing approximately 40% of women to avoid transvaginal ultrasound. To ensure high sensitivity of transabdominal screening, approximately 60% of patients will still require a transvaginal study.

      Key words

      Two randomized trials demonstrated that vaginal progesterone reduces the risk of spontaneous preterm birth in women with short midtrimester cervical length diagnosed by transvaginal (TV) ultrasound.
      • Fonseca E.B.
      • Celik E.
      • Parra M.
      • Singh M.
      • Nicolaides K.H.
      Progesterone and the risk of preterm birth among women with a short cervix.
      • Hassan S.S.
      • Romero R.
      • Vidyadhari D.
      • et al.
      Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
      However, the prevalence of a short cervix in a general obstetric population is low.
      • Fonseca E.B.
      • Celik E.
      • Parra M.
      • Singh M.
      • Nicolaides K.H.
      Progesterone and the risk of preterm birth among women with a short cervix.
      • Hassan S.S.
      • Romero R.
      • Vidyadhari D.
      • et al.
      Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
      • To M.S.
      • Alfirevic Z.
      • Heath V.C.
      • et al.
      Cervical cerclage for prevention of preterm delivery in women with short cervix: randomised controlled trial.
      Although recent decision analyses found universal TV ultrasound and treatment with vaginal progesterone to be cost effective,
      • Cahill A.G.
      • Odibo A.O.
      • Caughey A.B.
      • et al.
      Universal cervical length screening and treatment with vaginal progesterone to prevent preterm birth: a decision and economic analysis.
      • Werner E.F.
      • Han C.S.
      • Pettker C.M.
      • et al.
      Universal cervical-length screening to prevent preterm birth: a cost-effectiveness analysis.
      universal screening requires a significant dedication of resources that may not be available at all centers. Universal screening would entail performing millions of additional transvaginal studies annually in the United States.
      For Editors' Commentary, see Contents
      Some research studies have suggested that assessment of the cervix by transabdominal (TA) ultrasound may be a useful initial screening test to detect short cervix diagnosed by TV ultrasound
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      • Stone P.R.
      • Chan E.H.
      • McCowan L.M.
      • Taylor R.S.
      • Mitchell J.M.
      Transabdominal scanning of the cervix at the 20-week morphology scan: comparison with transvaginal cervical measurements in a healthy nulliparous population.
      thereby more efficiently identifying candidates for vaginal progesterone. However, other studies have found TA ultrasound screening to be a poor test for detecting short cervix (Table 1).
      • To M.S.
      • Skentou C.
      • Cicero S.
      • Nicolaides K.H.
      Cervical assessment at the routine 23-weeks' scan: problems with transabdominal sonography.
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      Disagreement in the literature regarding the sensitivity of TA screening may be secondary to the relatively small numbers of patients with short cervix included in each study because of the rare occurrence of this condition in an unselected population.
      TABLE 1Major recent studies comparing TA and TV ultrasonographic cervical length assessment
      StudyGestational ageNumber of patientsBladder status during TA assessmentMean TA cervical lengthMean TV cervical lengthMain findings
      Hernadez-Andrade et al
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      Mean: 24.4 wks

      Range: 6.3–39 wks
      220Prevoid34.6 mm34.8 mmTA ≤25 mm 43% sensitive for TV ≤25 mm.

      TA ≤30 mm 57% sensitive for TV ≤25 mm.
      Stone et al
      • Stone P.R.
      • Chan E.H.
      • McCowan L.M.
      • Taylor R.S.
      • Mitchell J.M.
      Transabdominal scanning of the cervix at the 20-week morphology scan: comparison with transvaginal cervical measurements in a healthy nulliparous population.
      Range: 18–20 wks203Postvoid36.6 mm39.1 mmIn discrepant cases, TA measurements ≤33 mm were shorter than TV measurements in 97% of cases.
      Saul et al
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      Range: 14–34 wks

      Mean: 22.2 wks
      191Postvoid35.7 mm36.1 mmTA ≤30 mm 100% sensitive for TV ≤25 mm.

      TA ≤33 mm 100% sensitive for TV ≤30 mm.
      To et al
      • To M.S.
      • Skentou C.
      • Cicero S.
      • Nicolaides K.H.
      Cervical assessment at the routine 23-weeks' scan: problems with transabdominal sonography.
      Range: 22–24 wks

      Mean: 23 weeks
      149Bladder volume calculated34 mm37 mmCervix visualized transabdominally in 49% of cases overall. Visualization more likely with increased bladder volume.
      TA, transabdominal; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      We hypothesized that there is a threshold cervical length measured by TA ultrasound above which the risk of short cervical length (≤25 mm) measured by TV ultrasound is extremely low. To test this hypothesis we determined (1) the proportion of women in whom maternal cervical length could be imaged adequately by TA ultrasound at 18 to 24 weeks' gestation; and (2) the test characteristics for TA ultrasound as a screening test to detect women with short cervix on TV ultrasound within a universal screening protocol. The identification of a TA cervical length threshold above which TV ultrasound does not have to be performed has the potential to reduce the clinical burden posed to prenatal ultrasound units by universal TV cervical length screening.

      Materials and Methods

      This prospective cohort study evaluated consecutive patients who underwent anatomy ultrasound in the maternal-fetal medicine division at the Hospital of the University of Pennsylvania between January 2012 and June 2012. Approval for this study was granted by the institutional review board of the University of Pennsylvania (protocol no. 815974). Cervical length measurement data were prospectively collected as part of a quality assurance initiative. Demographic and medical data were collected through review of electronic medical records.
      At the study center TV assessment of the cervix was offered as routine care during anatomy ultrasound. Patients were included if they were between 18 weeks' 0 days' and 23 weeks' 6 days' gestational age and had a singleton gestation at the time of examination. Patients beyond 23 weeks' 6 days' gestational age were excluded because prior studies have not evaluated the benefit of initiating vaginal progesterone beyond this gestational age.
      • Fonseca E.B.
      • Celik E.
      • Parra M.
      • Singh M.
      • Nicolaides K.H.
      Progesterone and the risk of preterm birth among women with a short cervix.
      • Hassan S.S.
      • Romero R.
      • Vidyadhari D.
      • et al.
      Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
      Two other categories of women were excluded: (1) women with cerclage already in place and (2) women with a prior spontaneous preterm birth already receiving 17-alpha hydroxyprogesterone caproate (17P) who were either not eligible or would not elect for cerclage if found to have a short cervix (based on the findings of Owen et al
      • Owen J.
      • Hankins G.
      • Iams J.D.
      • et al.
      Multicenter randomized trial of cerclage for preterm birth prevention in high-risk women with shortened midtrimester cervical length.
      ). Ultrasound is not offered to these patients because detection of a short cervix would not alter preterm birth prevention strategies. For women with a prior preterm birth who would undergo cerclage if found to have a short cervix, only the TV ultrasound performed during fetal anatomic survey at 18-24 weeks was included in this analysis.
      For each patient included in the study, prevoid and postvoid TA cervical length measurements were performed before TV ultrasound. Patients were given instructions during prenatal care to refrain from voiding in the 2 hours before their study. Pre- and postvoid measurement were obtained because some experts have expressed concern that bladder status may affect TA measurement.
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      Sonographers were instructed that if TA cervical length measurements were not readily attainable, the examination time of the study should not be extended to allow bladder filling and TA views should be noted as unobtainable.
      Before including patients in the study, sonographers received an in-service training program, which reviewed landmarks for assessing the cervix transabdominally based on the study by Saul et al.
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      Sonographers were instructed to obtain TA images in the midsagittal plane obtaining the following landmarks: the cervical/vaginal interface, the internal cervical os, the external cervical os, the outline of the cervical corpus, and the full length of the cervical canal. TA assessment of the cervix was initiated 1 month before the start of the protocol, and each sonographer was required to submit images from 5 patients demonstrating all landmarks. With the use of these landmarks as a guide, the sonographers were instructed during the study period to obtain the best possible image for TA cervical length assessment regardless of which landmarks were obtained. The TA cervical length was measured with a single linear measurement and rounded to the nearest millimeter. If a sonographer was unable to obtain an acceptable image, they were instructed to designate the TA cervical length “unobtainable.” TV ultrasound was performed according to standard practices.
      • Iams J.D.
      • Goldenberg R.L.
      • Meis P.J.
      • et al.
      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.
      FIGURE 1, FIGURE 2 demonstrate TA prevoid and TA postvoid ultrasound images respectively for a single patient.
      Figure thumbnail gr1
      FIGURE 1Prevoid TA cervical length
      TA, transabdominal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Figure thumbnail gr2
      FIGURE 2Postvoid TA cervical length
      TA, transabdominal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Demographic and medical information for each patient was collected from electronic medical records and included body mass index (BMI) calculated from first prenatal visit, gravidity and parity, major medical comorbidities, age, race, and whether the patient had undergone prior cesarean section and prior cervical surgery.
      The study was powered to allow for detection of a screen positive rate at which the sensitivity would be 95% (95% confidence interval [CI], 90–99%) for detecting women with short TV cervical length ≤25 mm. The anticipated prevalence for TV cervical length ≤25 mm was 6%. This estimate of short cervical length prevalence was chosen based on findings from our institution accounting for exclusion of women having already received 17P and/or cerclage. The number of patients needed for the analysis based on these assumptions was 1217.
      • Jones S.R.
      • Carley S.
      • Harrison M.
      An introduction to power and sample size estimation.
      A high sensitivity was chosen because false positive cases (performing unnecessary TV ultrasound studies) were preferred over false negative cases (missing women with short TV cervical length). The cutoff of 25 mm was used for our primary outcome because cervical length up to 25 mm is associated with increased preterm birth risk
      • Iams J.D.
      • Goldenberg R.L.
      • Meis P.J.
      • et al.
      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.
      and women with cervical length ≤25 mm may benefit from interventions such as cerclage if they have certain risk factors.
      • Berghella V.
      • Rafael T.J.
      • Szychowski J.M.
      • Rust O.A.
      • Owen J.
      Cerclage for short cervix on ultrasonography in women with singleton gestations and previous preterm birth: a meta-analysis.
      Because vaginal progesterone has only been shown to be beneficial up to a cervical length of 20 mm test statistics associated with this cutpoint are reported as secondary outcomes.
      • Hassan S.S.
      • Romero R.
      • Vidyadhari D.
      • et al.
      Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
      Secondary analyses also included generating receiver operating characteristic (ROC) curves to assess detection of women with TV cervical length of ≤25 mm and ≤20 mm. In addition, regression analysis was performed to determine whether demographic variables such as BMI at first prenatal visit, race, parity, and prior cesarean section were associated with increased absolute difference between TA and TV cervical length. Prevoid and postvoid TA cervical length measurements were compared to determine whether there was a significant difference in measurement based on bladder status. For all analyses, patients with unobtainable TA images were considered screen positive with a short measurement and would need to undergo TV as part of a screening protocol.
      All analyses were performed separately for both prevoid and postvoid measurements. However, primary and secondary outcomes are reported based on prevoid TA measurements; researchers have expressed concern that TA ultrasound with a full bladder may cause the cervix to appear falsely elongated. By using TA images with a full bladder, the study design is biased toward overestimation of TA cervical length and TA screening failing as a good screening test.
      STATA 10.0 was used to perform statistical analysis (StataCorp, College Station, TX). Categorical variables were studied using χ2 tests or Fisher exact as appropriate. Means of normally distributed continuous variables were studied using Student t tests or nonparametic statistics as appropriate. Paired data were accounted for using appropriate statistical methods. A P value of < .05 was considered significant for all analyses.

      Results

      There were 1349 patients with singleton pregnancy who met inclusion criteria and were entered into the study. A total of 132 women (9.8%) were excluded because they declined TV ultrasound; consequently, 1217 women underwent TA and TV cervical length screening and were included in our final analyses (Figure 3). In this cohort 6.2% of patients had TV cervical length ≤25 mm (n = 76) and 2.6% of patients (n = 32) had TV cervical length ≤20 (Table 2). The mean TV cervical length was 36.1 mm (SD 8.3 mm; 95% CI, 35.6–36.6 mm), the mean prevoid TA cervical length was 34.6 mm (SD 8.4 mm; 95% CI, 34.1–35.1 mm), and the mean postvoid TA cervical length was 33.5 mm (SD 8.4 mm; 95% CI, 33.0–34.1 mm). Both prevoid and postvoid mean TA cervical length were significantly shorter than mean TV cervical length (P < .01). A scatterplot demonstrates the relationship between TV and prevoid TA cervical length in Figure 4.
      Figure thumbnail gr3
      FIGURE 3Derivation of the study population
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      TABLE 2Demographics
      CharacteristicValue
      Age (mean, SD)28.2 (6.3)
      Race, %
       Black63.8
       White23.0
       Asian7.6
       Other4.3
       Unknown1.3
      Ethnicity, %
       Hispanic2.7
       Not Hispanic97.3
      Gestational age (mean, SD)20.5 (1.0)
      Nulliparous, %43.1
      BMI (mean, SD)27.8 (7.8)
      Medicaid, %58.7
      Medical comorbidity, %
       Hypertension5.1
       Pregestational diabetes2.0
       Renal disease0.4
       Other major medical comorbidity3.1
       Prior spontaneous preterm birth, %5.9
       History of cervical surgery, %3.0
       Uterine anomaly, %0.7
       TV CL ≤20 mm, n (%)32 (2.6)
       TV CL ≤25 mm, n (%)76 (6.2)
      BMI, body mass index; CL, cervical length; SD, standard deviation; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Figure thumbnail gr4
      FIGURE 4Scatterplot demonstrating relationship between TV and TA cervical length
      Horizontal lines represent TV cutoffs of 20 and 25 mm.
      TA, transabdominal; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Prevoid TA ultrasound was 96.1% sensitive at a cutoff of ≤36 mm for detecting short cervix on TV ultrasound of ≤25 mm (95.0% CI, 90.0–99.2%) with a specificity of 39.4% (95% CI, 36.7–42.2%) (Table 3). The area under the ROC for TV cervical length ≤25 mm was 0.76 (Figure 5). Prevoid TA ultrasound was 100% sensitive at a cutoff of ≤35 mm for detecting short cervix on TV ultrasound ≤20 mm (95% CI, 89.1–100.0%). Specificity was 40.8% (95% CI, 38.0–43.7%) at this cutoff (Table 4). The area under the ROC for TV cervical length ≤20 mm was 0.86 (Figure 6).
      TABLE 3Test characteristics of prevoid TA ultrasound for detection of cervical length ≤25 mm
      CharacteristicTA ≤25 mmTA ≤30 mmTA ≤32 mmTA ≤36 mm
      Sensitivity (95% CI)44.7% (33.3–56.6%)72.4% (60.9–82.0%)78.9% (68.1–87.5%)96.1% (90.0–99.2%)
      Specificity (95% CI)84.2% (82.0–86.3%)65.6% (62.8–68.4%)55.5% (52.5–58.4%)39.4% (36.7–42.2%)
      Negative predictive value (95% CI)95.8% (94.6–97.0%)97.3% (96.2–98.4%)97.5% (96.3–98.7%)99.3% (98.5–99.9%)
      Positive predictive value (95% CI)15.9% (11.6–21.4%)12.3% (9.6–15.7%)10.5% (8.3–13.3%)9.3% (7.4–11.5%)
      If the cervix could not be viewed on transabdominal view it was considered screen positive and a value of <25 mm on transabdominal length was assigned for all test characteristic calculations.
      CI, confidence interval; TA, transabdominable.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Figure thumbnail gr5
      FIGURE 5ROC curve for prevoid TA screening for patients with short cervix on TV ultrasound ≤25 mm
      The triangle, circle, and rectangle represent cutoffs of 25 mm, 30 mm, and 36 mm on TA ultrasound.
      ROC, receiver operating characteristic; TA, transabdominal; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      TABLE 4Test characteristics of prevoid TA ultrasound for detection of cervical length ≤20 mm
      CharacteristicTA ≤25 mmTA ≤30 mmTA ≤32 mmTA ≤35 mm
      Sensitivity (95% CI)71.9% (51.3–86.3%)90.6% (75.0–98.0%)90.6% (75.0–98.0%)100% (89.1–100%)
      Specificity (95% CI)83.9% (81.7–85.9%)64.7 (62.0–67.4%)55.5% (52.0–57.9%)40.8% (38.0–43.7%)
      Negative predictive value (95% CI)99.1% (98.5–99.7%)99.6% (98.9–99.8%)99.5% (98.6–99.8%)100% (99.2–100%)
      Positive predictive value (95% CI)11.2% (7.7–16.1%)6.5% (4.6–9.2%)5.1% (3.6–7.3%)4.5% (3.2–6.3%)
      CI, confidence interval; TA, transabdominal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      Figure thumbnail gr6
      FIGURE 6ROC curve for prevoid TA screening for patients with short cervix on TV ultrasound ≤20 mm
      The triangle, circle, and rectangle represent cutoffs of 25 mm, 30 mm, and 35 mm on TA ultrasound.
      ROC, receiver operating characteristic; TA, transabdominal; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      In our cohort, using >35 mm as a prevoid TA cutoff would result in 39.8% of patients avoiding TV ultrasound. The 60.2% of patients would still require TV ultrasound either because their TA length was ≤35 mm or because their cervix could not be viewed transabdominally.
      Images of the cervix were unobtainable in 6.2% of patients prevoid and 17.9% of patients postvoid. Although postvoid TA assessment demonstrated similar sensitivity at TA cutpoints in detecting short TV cervical (35 mm on postvoid TA ultrasound was 96.9% sensitive for TV ≤20 mm and 96.1% sensitive for TV ≤25 mm), specificity was lower as the screen positive rate was higher compared with prevoid assessment because of the larger proportion of images that were unobtainable postvoid (TABLE 5, TABLE 6).
      TABLE 5Test characteristics of postvoid TA ultrasound for detection of cervical length ≤25 mm
      CharacteristicTA ≤25 mmTA ≤30 mmTA ≤32 mmTA ≤35 mm
      Sensitivity (95% CI)61.8% (50.6–71.9%)77.6% (67.1–85.5%)88.2% (79.0–93.6%)96.1% (89.0–98.7%)
      Specificity (95% CI)73.7% (71.1–76.2%)53.9% (51.0–56.8%)42.9% (40.1–45.8%)29.7% (27.1–32.4%)
      Negative predictive value (95% CI)96.7% (95.2–97.7%)97.3% (95.7–98.3%)98.2% (96.6–99.1%)99.1% (97.5–99.7%)
      Positive predictive value (95% CI)13.5% (10.2–17.7%)10.1% (7.9–12.9%)9.3% (7.4–11.7%)8.3% (6.6–10.4%)
      CI, confidence interval; TA, transabdominal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      TABLE 6Test characteristics of postvoid TA ultrasound for detection of cervical length ≤20 mm
      CharacteristicTA ≤25 mmTA ≤30 mmTA ≤32 mmTA ≤35 mm
      Sensitivity (95% CI)75.0% (57.9–87.9%)84.4% (68.3–93.1%)93.75% (79.9–98.3%)96.9% (84.3–99.5%)
      Specificity (95% CI)72.7% (70.1–75.2%)52.9% (50.1–55.7%)41.9% (39.2–44.8%)28.4% (25.9–31.1%)
      Negative predictive value (95% CI)99.1% (98.2–99.5%)99.2% (98.2–99.7%)99.6% (98.6–99.9%)99.7% (98.3–99.9%)
      Positive predictive value (95% CI)6.9% (4.6–10.3%)4.6% (3.1–6.7%)4.2% (2.9–5.9%)3.5% (2.5–5.0%)
      CI, confidence interval; TA, transabdominal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.
      The discrepancy in millimeters between prevoid TA and TV cervical length was calculated for each patient. BMI, race, parity, insurance status, age, and prior cesarean section were not associated with a difference in the mean discrepancy between TA and TV cervical length. The mean discrepancy between TV and TA cervical length was similar in women with underweight, normal, overweight, and obese BMIs (Table 7).
      TABLE 7Mean TA-TV discrepancy by BMI
      BMI% of patientsMean discrepancy
      ≤208.47.6 mm
      >20-2539.17.2 mm
      >25-3023.27.1 mm
      >30-4021.57.1 mm
      >407.47.0 mm
      BMI, body mass index; TA, transabdominal; TV, transvaginal.
      Friedman. Transabdominal ultrasound as a screening test for short cervix. Am J Obstet Gynecol 2013.

      Comment

      Our results demonstrate that TA screening may significantly reduce the burden of universal cervical length screening in prenatal ultrasound units. At cutpoints that allowed exclusion of approximately 40% of negative TV ultrasounds, TA screening allowed detection of 100% of patients with TV cervical length ≤20 mm and >95% of patients with TV cervical length ≤25 mm in our cohort. Acceptance of TA screening may encourage more centers to offer routine cervical length assessment by making such services more feasible. Our results suggest that in a hypothetical ultrasound unit performing 2500 anatomy ultrasounds annually, using a TA prevoid cutpoint of >35 mm would result in approximately 1000 women avoiding TV cervical length. Approximately 1500 women (or 60% of patients) would still require TV cervical length measurement.
      Several aspects of this study enhance its potential clinical applicability. First, sonographers were instructed not to increase anatomy ultrasound study time to obtain TA cervical length images to allow bladder filling. Despite these constraints, prevoid TA cervical length measurements were still able to be obtained in 94% of cases. Second, the majority of patients in this population were overweight, obese, or morbidly obese. Our results demonstrate that TA screening was successful in a majority overweight population. Third, image acceptability was at the discretion of the sonographer, suggesting that TA evaluation of the cervix is readily interpretable. A weakness of this study is that although no differences in TA performance were found based on demographic variables, the majority of our population was Medicaid insured and African-American, limiting generalizability. Another weakness of this study is that the same sonographer obtained both TA and TV views for each patient. A future, confirmatory study could use individual blinded sonographers to separately obtain TA and TV images to eliminate this source of potential bias.
      Prior research has demonstrated conflicting results regarding the usefulness of TA screening. Although Saul et al
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      found 30 mm to be a sensitive TA cutpoint for detecting TV cervical length ≤25 mm, Hernandez-Andrade et al
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      found TA measurements less than 25 and 30 mm to be poorly sensitive. Our results at lower cutoffs were similar to those of Hernandez-Andrade et al
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      and indicated that higher TA cutoffs are required to achieve high sensitivity. Disagreement between previous studies may be due to study designs that focused on describing the correlation between TA and TV ultrasound and were underpowered to determine test characteristics for prediction of short cervical length at ≤25 and ≤20 mm on TV ultrasound. Although identification of women with short cervix at ≤20 mm is critically important because these women may benefit from vaginal progesterone as demonstrated by the studies by Fonseca et al
      • Fonseca E.B.
      • Celik E.
      • Parra M.
      • Singh M.
      • Nicolaides K.H.
      Progesterone and the risk of preterm birth among women with a short cervix.
      and Hassan et al,
      • Hassan S.S.
      • Romero R.
      • Vidyadhari D.
      • et al.
      Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
      the prevalence of these women is relatively low and a large sample size is required to calculate meaningful test statistics. Critics of TA screening have expressed concern that TA ultrasound may overestimate cervical length in the setting of true short cervix
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      ; although in some cases significant discrepancy did exist between TA and TV cervical length measurement within our cohort, this did not preclude TA screening from demonstrating high sensitivity at cutpoints that resulted in significant numbers of screen negative women in this study. In addition, the mean TA cervical length both pre- and postvoid was shorter than mean TV cervical length. These findings agree with those of other studies that found mean TA cervical length is shorter than or similar to mean TV cervical length.
      • Saul L.L.
      • Kurtzman J.T.
      • Hagemann C.
      • Ghamsary M.
      • Wing D.A.
      Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
      • Stone P.R.
      • Chan E.H.
      • McCowan L.M.
      • Taylor R.S.
      • Mitchell J.M.
      Transabdominal scanning of the cervix at the 20-week morphology scan: comparison with transvaginal cervical measurements in a healthy nulliparous population.
      • To M.S.
      • Skentou C.
      • Cicero S.
      • Nicolaides K.H.
      Cervical assessment at the routine 23-weeks' scan: problems with transabdominal sonography.
      • Hernandez-Andrade E.
      • Romero R.
      • Ahn H.
      • et al.
      Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
      An important consideration in interpreting the results of this study is that to obtain a high sensitivity of short cervix detection approximately 60% of women will still need a TV ultrasound after TA screening. This is in part because of unobtainable prevoid images in a minority of case (6.2%). Often prevoid images were unobtainable secondary to poor visualization of the lower uterine segment or a fetal bony structure obscuring imaging of the cervix. In postvoid patients, the rate of unobtainable images was considerably higher (17.9%), secondary to the absence of the acoustic window provided by the bladder. If postvoid TA screening is used, an even higher percentage of women will need TV studies. Another important consideration in interpreting these results clinically is that ultrasonographers underwent specific education and training before the start of the study and that TA screening may perform less well if sonographers do not undergo similar training.
      Adoption of universal cervical length assessment is currently being considered as a preterm birth prevention strategy.
      • Parry S.
      • Simhan H.
      • Elovitz M.
      • Iams J.
      Universal maternal cervical length screening during the second trimester: pros and cons of a strategy to identify women at risk of spontaneous preterm delivery.
      Guidelines from the Society for Maternal-Fetal Medicine state that although universal CL screening remains controversial, “implementation of such a screening strategy can be viewed as reasonable, and can be considered by individual practitioners.”
      Society for Maternal-Fetal Medicine Publications Committee
      Progesterone and preterm birth prevention: transalting clinical trials data into clinical practice.
      Our results support the clinical usefulness and feasibility of both universal TV ultrasound cervical length measurement and TA screening. In our cohort, the majority of women identified (87.1%) with short cervix ≤20 mm had no clinical risk factors such as prior preterm birth. In addition, greater than 90% percent of patients elected for TV screening when offered. Further research to assess cost-effectiveness of TA cervical length screening is needed. In conclusion, although universal TV ultrasound is acceptable to the vast majority of patients, TA assessment of the cervix may be useful as a highly sensitive screening test to help reduce the burden posed to prenatal ultrasound centers by universal cervical length screening.

      Acknowledgments

      We would like to thank the following sonographers for their work on this study: Deb Riesch, Sue Betsch, Cynthia Ross, Denenne Paquin, and Carmen Fund.

      References

        • Fonseca E.B.
        • Celik E.
        • Parra M.
        • Singh M.
        • Nicolaides K.H.
        Progesterone and the risk of preterm birth among women with a short cervix.
        N Engl J Med. 2007; 357: 462-469
        • Hassan S.S.
        • Romero R.
        • Vidyadhari D.
        • et al.
        Vaginal progesterone reduces the rate of preterm birth in women with a sonographic short cervix: a multicenter, randomized, double-blind, placebo-controlled trial.
        Ultrasound Obstet Gynecol. 2011; 38: 18-31
        • To M.S.
        • Alfirevic Z.
        • Heath V.C.
        • et al.
        Cervical cerclage for prevention of preterm delivery in women with short cervix: randomised controlled trial.
        Lancet. 2004; 363: 1849-1853
        • Cahill A.G.
        • Odibo A.O.
        • Caughey A.B.
        • et al.
        Universal cervical length screening and treatment with vaginal progesterone to prevent preterm birth: a decision and economic analysis.
        Am J Obstet Gynecol. 2010; 202: 548.e1-548.e8
        • Werner E.F.
        • Han C.S.
        • Pettker C.M.
        • et al.
        Universal cervical-length screening to prevent preterm birth: a cost-effectiveness analysis.
        Ultrasound Obstet Gynecol. 2011; 38: 32-37
        • Saul L.L.
        • Kurtzman J.T.
        • Hagemann C.
        • Ghamsary M.
        • Wing D.A.
        Is transabdominal sonography of the cervix after voiding a reliable method of cervical length assessment?.
        J Ultrasound Med. 2008; 27: 1305-1311
        • Stone P.R.
        • Chan E.H.
        • McCowan L.M.
        • Taylor R.S.
        • Mitchell J.M.
        Transabdominal scanning of the cervix at the 20-week morphology scan: comparison with transvaginal cervical measurements in a healthy nulliparous population.
        Aust N Z J Obstet Gynaecol. 2010; 50: 523-527
        • To M.S.
        • Skentou C.
        • Cicero S.
        • Nicolaides K.H.
        Cervical assessment at the routine 23-weeks' scan: problems with transabdominal sonography.
        Ultrasound Obstet Gynecol. 2000; 15: 292-296
        • Hernandez-Andrade E.
        • Romero R.
        • Ahn H.
        • et al.
        Transabdominal evaluation of uterine cervical length during pregnancy fails to identify a substantial number of women with a short cervix.
        J Matern Fetal Neonatal Med. 2012; 25: 1682
        • Owen J.
        • Hankins G.
        • Iams J.D.
        • et al.
        Multicenter randomized trial of cerclage for preterm birth prevention in high-risk women with shortened midtrimester cervical length.
        Am J Obstet Gynecol. 2009; 201: 375.e1-375.e8
        • Iams J.D.
        • Goldenberg R.L.
        • Meis P.J.
        • et al.
        The length of the cervix and the risk of spontaneous premature delivery.
        N Engl J Med. 1996; 334: 567-572
        • Jones S.R.
        • Carley S.
        • Harrison M.
        An introduction to power and sample size estimation.
        Emerg Med J. 2003; 20: 453
        • Berghella V.
        • Rafael T.J.
        • Szychowski J.M.
        • Rust O.A.
        • Owen J.
        Cerclage for short cervix on ultrasonography in women with singleton gestations and previous preterm birth: a meta-analysis.
        Obstet Gynecol. 2011; 117: 663-671
        • Parry S.
        • Simhan H.
        • Elovitz M.
        • Iams J.
        Universal maternal cervical length screening during the second trimester: pros and cons of a strategy to identify women at risk of spontaneous preterm delivery.
        Am J Obstet Gynecol. 2012; 207: 101-106
        • Society for Maternal-Fetal Medicine Publications Committee
        Progesterone and preterm birth prevention: transalting clinical trials data into clinical practice.
        Am J Obstet Gynecol. 2012; 206: 376-386