Objective: The aim of the study was to describe changes in the acceptance of transvaginal (TV) cervical length (CL) assessment and in the variance of CL measurements among operators, after implementation of universal TV-CL screening at 18+0 – to 23+6 weeks/days of gestation. Design: Retrospective cohort. Participants/Materials, Setting, Methods: This study was performed after universal TV-CL screening was implemented at the University of Texas Health Science Center in Houston, TX, USA, for all women undergoing an anatomy ultrasound (US) between 18 0/6 and 23 6/7 weeks/days of gestation. Pregnant women carrying singletons without prior history of preterm delivery who underwent anatomy US evaluation between September 2017 and March 2020 (30 months) were included. The complete study period was divided into five epochs of 6 months each. Changes in patient’s acceptance for the TV scan, in CL distribution, in the prevalence of short cervix defined as ≤15, ≤20, or ≤25 mm, and in the performance of US operators across the five epochs were evaluated. Success rate was defined as the percentage of TV-CL measurements obtained in relation to the number of second-trimester anatomy scans. Results: A total of 22,207 low-risk pregnant women evaluated by 36 trained sonographers (operators) were analyzed. Overall, the acceptance for TV-CL measurement was 82.3% (18,289/22,207), increasing from 76.7% in the first epoch to 82.8% (p < 0.0001) in the last epoch. The mean CL did not significantly change from 38.6 mm in the first epoch to 38.5 mm in the last epoch (p = 0.7); however, the standard deviation decreased from 7.9 mm in the first epoch to 7.04 mm in the last epoch (p = <0.01). The prevalence of a short cervix ≤25 mm was 2.2% (n = 399/18,289), ≤20 mm was 1.2% (224/18,289), and ≤15 mm was 0.9% (162/18,289). This prevalence varied only for CL ≤25 mm from 3.02% (88/2,907) in the first epoch to 1.77% (64/3,615) in the last epoch (p = 0.0009). There was a variation in CL measurements among operators (mean 3.3 mm). Sonographers with less than 1 year of experience had a lower success rate for completing TV-CL examinations than more experienced sonographers (80.8% vs. 85.8%; p < 0.03). In general, 77% (27/35) of operators had a success rate ≥80% for completing TV-CL scans. Limitations: Characteristics of individuals who accepted versus those who declined TV-CL were not compared; CL values were not correlated with clinical outcomes. Conclusions: During the first 6 months after implementation of a universal CL screening program, there was greater variation in CL measurements, lower acceptance for TV US, and a higher number of women diagnosed with a CL ≤25 mm, as compared to subsequent epochs. After the first 6 months, these metrics improved and remained stable. Most operators improved their performance over time; however, there were a few with a low success rate for TV-CL and others who systematically over- or underestimate CL measurements.

Universal screening for identification of women at risk of preterm delivery by measuring the cervical length (CL) with transvaginal (TV) ultrasound (US) during the second-trimester US scan has proven to be cost effective [1‒4]. Women with a short cervix without history of previous preterm births benefit from treatment with vaginal progesterone by reducing the incidence of preterm delivery <35 weeks of gestation and of neonatal complications [5‒8]. However, aspects challenging universal TV-CL screening include increased costs, prolonged scanning time, patient’s discomfort, and a low positive predictive value [9‒11]. An alternative approach is screening based on risk factors, considering not only obstetric history, but maternal characteristics such as ethnicity, tobacco use, and maternal weight [12]. Whereas this strategy may significantly reduce the number of CL examinations, it may not detect the same number of women at risk of preterm delivery as compared to universal TV-CL screening.

The definition of a short cervix varies from ≤30, ≤25, ≤20, or ≤15 mm, and the time of evaluation ranges from 16 to 24 weeks of gestation [5, 6, 13‒15]. Variations in the definition and prevalence of short cervix and preterm delivery in addition to increased costs and prolongation of the scanning time, are considered potential limitations for universal TV-CL  screening in all populations [16‒19].

Implementation of such a screening program in a large medical system should consider all the above-mentioned factors. Perhaps, the most foundational aspect is training and supervision, which addresses the uniformity and accuracy of CL measurements [20]. Training may also foster conviction in US operators of the benefits of identifying women with a short cervix, enable them to communicate these benefits to patients, and assist them in performing the highest quality measurement possible.

In our medical system, we implemented a universal TV-CL screening program in the fall of 2017. In this report, we describe changes over time in the acceptance of the TV scan among patients, in the distribution of CL values, and in the impact US operators have on CL measurements.

The present analysis was performed in a 30-month period between September 2017 and March 2020, divided into five epochs of 6 months each. Pregnant women attending our Maternal-Fetal Medicine (MFM) US units for their routine second-trimester anatomy scan were included in this study. Only data obtained from women with singleton pregnancies between 18 0/7 and 23 6/7 weeks/days of gestation were analyzed. Exclusion criteria were history of spontaneous preterm delivery or preterm premature rupture of membranes, twin or multiple gestation, a known short cervix, and cerclage in place, or clinical symptoms suggesting preterm labor, i.e., contractions, abdominal pain, TV bleeding, or leaking of fluid. Individuals were informed about the potential benefits of TV-CL measurement by the US operator and had the option to accept or decline the scan. If an individual declined TV-CL, it was noted on the US report contemporaneously.

Informed consent was waived by the institutional review board from The University of Texas McGovern Medical School (HSC-MS-20-1050). The University of Texas McGovern Medical School Department of Obstetrics and Gynecology provides MFM coverage to ten US units in Houston and Houston greater area. Each unit has 1–4 US machines operated by trained sonographers (operators) and supervised by a board-certified or board-eligible MFM faculty. All US sonographers are Registered Diagnostic Medical Sonographer certified in obstetrics and were Cervical Length Education and Review (CLEAR) certified prior to implementation of universal TV-CL screening [20]. At the time of this study, all sonographers were female. The CLEAR program integrates an online course with examination and image review for TV-CL measurement. In addition, all US operators received formal lectures on TV-CL prior to implementation, a script on how to talk to patients about the use of TV-CL, and constant feedback by the MFM faculty. If a patient had questions about TV US, the MFM specialist covering the clinic was available to discuss additional information. To ensure a minimum level of expertise, we only included data from operators performing at least 100 CL measurements.

US Examinations

US evaluations were performed with GE Voluson E-8 systems with 2–6 MHz curved linear abdominal and 12-9 MHz TV US probes. All patients had a transabdominal scan for detailed evaluation of the fetal anatomy and biometry, and were informed of the potential benefit of CL measurement and management options in case of having a short cervix. The sonographer explained in detail the TV-CL procedure and potential discomfort patients may refer.

TV-CL was measured following CLEAR guidelines for TV US [20]: empty or near-empty bladder, identification of the mid-sagittal plane of the cervix, cervix must occupy approximately 75% of the image, clear view of the internal and the external cervical os and of the endocervical canal, and similar thickness of the anterior and posterior cervical lips. Calipers were placed from the internal to the external os following the endocervical canal, and CL traced and measured by subsequent points. At least three measurements without and with fundal pressure were obtained, and the shortest was considered representative of the patient and used for clinical care. For analysis of prevalence, different CL cut-offs (≤25.0, ≤20.0, ≤15.0 mm) for short cervix were used.

Analysis

Changes in patient’s acceptance toward the TV-CL measurement and in the prevalence of short cervix across the five epochs were analyzed. Acceptance of TV US was defined when the anatomy scan had a concurrent TV-CL measurement. Differences in the distribution and variance of CL measurements among US operators performing at least 100 TV-CL measurements were estimated. The success rate of completing the TV-CL scan was evaluated in relation to the experience of the operator and to changes over time in the proportion of TV-CL measurements obtained during the study period. Differences in mean CL values among the different epochs were calculated using one-way ANOVA and post hoc tests, and changes in variance using the Levene test for homogeneity. Differences in proportions were evaluated with Kruskal-Wallis and Fisher exact tests. A p value <0.05 was considered significant. No power calculation was performed as we aimed to describe changes over time in the complete population. Analyses were performed with SPSS® 19 (IBM Corp., Armonk, NY, USA) statistical software.

Overall

A total of 22,207 women with singleton pregnancies were evaluated by 36 trained sonographers in a 30-month period. Characteristics of the study group are shown in Table 1; 48.1% (10,676/22,207) women had overweight, and 26.9% were obese (5,968/22,207).

Table 1.

Characteristics of the study population

 Characteristics of the study population
 Characteristics of the study population

Acceptance of TV-CL and Variance of CL Values

The overall patient’s acceptance for a TV-CL measurement was 82.3% (18,289/22,207), increasing from 76.7% (2,907/3,789) in the first epoch to 82.8% (3,615/4,367; p < 0.0001) in the last epoch (Table 2). There were subtle variations in the acceptance rates among different epochs, but rates remained above 80% for most of the study period (Table 2).

Table 2.

Rate of TV-CL measurements according to the total number of anatomy scans performed during each epoch

 Rate of TV-CL measurements according to the total number of anatomy scans performed during each epoch
 Rate of TV-CL measurements according to the total number of anatomy scans performed during each epoch

CL Measurements

The mean CL did not significantly change from 38.6 mm in the first epoch to 38.5 mm in the last epoch (p = 0.7) (Table 2; Fig. 1); however, the standard deviation showed a significant reduction from 7.9 mm in the first to 7.04 mm in the last epoch (p < 0.01). The Levene test for homogeneity showed significant variance of CL measurements being higher in the first as compared to all other epochs (p < 0.0001; Table 2).

Fig. 1.

CL distribution values across the five different epochs.

Fig. 1.

CL distribution values across the five different epochs.

Close modal

Prevalence of a Short Cervix

The overall prevalence of a short cervix defined as ≤25 mm was 2.2% (399/18,289); of that defined as ≤20 mm was 1.2% (224/18,289); and of that defined as ≤15 mm was 0.9% (162/18,289) (Table 3). This prevalence only varied over time for CL ≤25 mm from 3.02% (88/2,907) to 1.77% (64/3,615) between epochs 1 and 5 (p = 0.001). No differences in the prevalence of CL ≤20 mm or ≤15 mm were observed among the different epochs.

Table 3.

Prevalence of short cervix defined by different CL cut-off values in relation to the different epochs

 Prevalence of short cervix defined by different CL cut-off values in relation to the different epochs
 Prevalence of short cervix defined by different CL cut-off values in relation to the different epochs

Variation among Operators

The average number of CL measurement obtained by 36 US operators was 516 (range 106–1,061). There was a 7.28 mm variation between the minimum and maximum mean CL values among operators (p < 0.001) (Table 4). There was no difference in the average number of US scans among operators who measured above the overall mean (n = 465) as compared to those who measured below the overall mean (n = 567; p = 0.2).

Table 4.

Individual variances among operators (n = 36) performing at least 100 CL measurements

 Individual variances among operators (n = 36) performing at least 100 CL measurements
 Individual variances among operators (n = 36) performing at least 100 CL measurements

The average years of experience among US operators were 8.8 years ranging from <1 to 36 years. Years of experience were not different between sonographers with mean CL values either above or below the overall CL mean. Operators who measured above the overall mean had an average of 9.9 years of experience (6 with ≤1 year of experience), whereas those who measured below the overall mean had an average of 7.7 years of experience (p = 0.52); among them, four had ≤1 year of experience. There was a significant difference in the success rate for performing TV-CL scans according to the experience of the operators (Fig. 2). Operators having <1 year of experience had a lower rate of TV-CL scans (80.8% [56.4–92.3%]) than those with more than 10 years of experience (85.8% [62.3–97.2%]; p = 0.03).

Fig. 2.

Success rate for obtaining TV-CL measurements according to years of experience of the sonographers.

Fig. 2.

Success rate for obtaining TV-CL measurements according to years of experience of the sonographers.

Close modal

Analysis of the individual performances of 36 sonographers (Fig. 3) showed that 13/35 sonographers (37.1%) had a consistent success rate of TV-CL ≥80%, whereas 14 (40.0%) significantly improved over time, reaching a success rate ≥80%. Seven (20%) operators constantly had a low success rate for obtaining TV-CL scans (<80%), but only one operator significantly reduced her success rate for obtaining TV-CL measurements. Changes over time were not analyzed for one operator as she worked only in one epoch.

Fig. 3.

Proportion of US operators having a good or low success rate for obtaining TV cervical measurements during the study period.

Fig. 3.

Proportion of US operators having a good or low success rate for obtaining TV cervical measurements during the study period.

Close modal

The main findings of our study are as follows: (1) during the first 6 months after implementing universal CL screening, a lower acceptability for the TV scan, a higher variation of the CL measurements, and an increased frequency of CLs ≤25 mm were observed as compared to subsequent periods; (2) despite similar training, experience, and supervision, there was a significant variation in CL measurements among sonographers (operators). There were sonographers who systematically underestimated and sonographers who systematically overestimated the CL measurement across all epochs; (3) more experienced sonographers had a significantly higher success rate for obtaining TV-CL measurement than less experienced ones; (4) most sonographers improved their success rate for obtaining TV-CL measurements over time; however, some never improved. A periodic evaluation of these metrics can contribute to identify systematic variations and apply actions to improve sonographers’ performance and reduce the variability of CL measurements when a universal CL screening program is implemented.

Variation among US Operators

External validation of studies establishing clinical standards is clinically useful. In randomized control trials, variables are under control, and examinations performed under high-quality standards [21, 22]. In clinical facilities under less controlled conditions, such high predictive or diagnostic values may not be reached as different factors can affect measurements and results.

Accuracy of CL measurements is strongly associated with the expertise of US operators; however, and despite having similar training, a significant variation in CL measurements among operators has been reported [13, 23]. Kuusela et al. [24] described a mean difference of 0.33 mm with wide 95% limits of agreement (−4.06 to 4.72 mm) when experienced operators obtained paired CL measurements from the same patients. Similar results were described by Valentin and Bergelin [25] who reported wide limits of agreement (±10 mm) when two experienced operators with similar training obtained paired CL measurements from the same patients. The authors suggested that a variation of ±5 mm in CL might be clinically acceptable, but a 10 mm variation might have an important impact on the clinical management.

We did not find differences in years of experience among sonographers measuring either above or below the overall CL mean. Experience contributed to perform more TV-CL examinations, but not to reduce the variability among sonographers. Other variables such as physiological changes or/and contractions might account for CL variations [26, 27]; however, the main source of error is the sonographer’s technique. Filce et al. [28] proposed an auditable program to improve the reliability of CL measurements consisting in creating “optimal” mean and dispersion values. Data from each US operator can be audited for evaluation of the individual variability. Using this approach, the authors reported an improvement over time in the repeatability of CL measurements. Pedretti et al. [29] also suggested using a more consistent accreditation process for operators obtaining TV-CL measurements. Nevertheless, and in accordance with our results, a variation in CL measurements should be expected among US operators despite having similar training, expertise, and supervision.

Acceptance of the TV US Scan

Differences in patient’s perception toward TV US may also be related to the sonographer. According to our results, during the first 6 months of universal CL measurement implementation, there was a lower acceptance rate of the TV scan as compared to subsequent periods. The addition of a TV scan might be perceived as a change in the normal workflow with prolongation of the scanning time. Later, this procedure became part of the routine evaluation and sonographers are more proactive in proposing and measuring the TV-CL. Similar to our results, Temming et al. [30] reported an increase in patient’s acceptance to TV US scan from 77.4% in 2011, when their program started, to 87.8% 4 years later, with no differences in TV US acceptance from the second period-evaluated (2012) onward. The effect of measuring the cervix on the total scanning time was evaluated in a study comparing three different strategies: (1) transabdominal CL measurement and if needed TV US; (2) TV US with three CL measurements; and (3) TV US with only one CL measurement. The authors did not find significant differences in the total scanning time among the three different strategies; only the time spent for cleaning the US probes was longer when a TV US was performed [31].

Nevertheless, a 100% acceptance rate may be difficult to achieve as other factors such as a perception of discomfort may influence the patient’s decision. In this regard, Dutta and Economides [32] applied a questionnaire to 107 pregnant women exploring their perception after a TV US scan. They reported 1.9% of women referring discomfort and 1% embarrassment during a TV US. There was a higher acceptance for the TV scan among nulliparous women than among multiparous women. Nevertheless, 99% of all women agreed on the possibility of having a TV US in subsequent clinical evaluations. Temming et al. [30] also reported that declining TV-CL was more common in women from Hispanic and African American origin, as well as those who were obese, multiparous, and tobacco users. Garthey et al. [33] suggested that patient’s acceptance to TV US can also be related to the sonographer’s perception. They reported a significant difference in patient’s acceptance when different sonographers with similar experience were evaluated. The rate of patient’s acceptance related to each sonographer did not change over time, reflecting the perceived idea of the sonographer toward the potential benefit of TV-CL measurement. We also observed this pattern, as an important number of sonographers had either a constant high or a low success rate for obtaining TV-CL measurements since the universal CL screening program was implemented. Others improved their performance over time; unfortunately, a few reduced their success rate for TV-CL measurements over time. Other factors such as insurance and out-of-pocket cost to the patient may also influence the decision of the patient to accept or decline a TV US. Unfortunately, we are unable to assess objectively the reasons for our patients to decline TV-CL screening.

Clinical Implications

Implementing a Universal CL Screening Program

The main purpose of a universal TV-CL screening program is to identify women with a short cervix who are at a higher risk of preterm delivery at the time of the second-trimester US scan; these patients benefit from treatment with vaginal progesterone [34]. This potential benefit relies on the prevalence of a short cervix and preterm delivery among different populations, which indeed can be different [18], whereas the mean CL seems to be similar among different groups, the distribution of CL values, and therefore, the definition of a short cervix varies. Furthermore, still using the same cut-off CL value, there are significant differences in the prevalence of short cervix among populations [17]. Nevertheless, cost-benefit studies have shown that universal screening and preventive treatment with vaginal progesterone are beneficial if the prevalence of short cervix (defined as <15 mm) is at least 0.35% [35]. Einerson et al. [12] concluded that there are more benefits of screening than of no-screening, despite the need of additional US scans for an apparently low detection rate of short cervix and preterm deliveries. Most studies evaluating costs and benefits of universal CL screening take into account the economic impact of early deliveries and neonatal costs against TV US and progesterone treatment [1]. Due to all these factors, several national guidelines only recommend the measurement of CL in women at risk instead of supporting implementation of universal CL screening for all patients [36‒39]. We and others believe that universal TV US CL is important and should be executed with systematic training and supervision [40, 41].

Strengths and Weaknesses

The strengths of our study are as follows: (1) evaluation of TV-CL measurements over defined periods of time after implementation of the universal screening program; (2) analysis of data obtained from a large number of US operators having similar training and evaluation; (3) the use of different cut-off values to define a short cervix; (4) identification of individual trends among US operators. Our main weaknesses were as follows: (1) we do not have maternal characteristics of individuals accepting or not a TV-CL scan; (2) CL values were not correlated with clinical outcomes; and (3) we did not apply a questionnaire evaluating why patients rejected TV US.

Our results indicate that after implementation of a CL universal screening variations in the success rate for obtaining TV-CL measurements should be expected, mainly during the first months. Periodic analysis, continuous training, and audit might contribute to identify possible sources of error addressing those factors individually.

We would like to thank all Registered Diagnostic Medical Sonographer who contributed in the implementation and establishment of the universal TV-CL screening program in The University of Texas McGovern Medical School Department of Obstetrics and Gynecology.

Informed consent was waived by the institutional review board from The University of Texas McGovern Medical School (HSC-MS-20-1050). Data evaluation complies with the guidelines for human studies, and data collection was conducted ethically in accordance with the World Medical Association and the Declaration of Helsinki.

The authors declare no conflicts of interest.

No funding sources were required for the preparation of this manuscript.

Eleazar E. Soto and Edgar Hernandez-Andrade: study design, data collection and analysis, and drafting and reviewing the manuscript. Erin S. Huntley: data collection and analysis, and drafting and reviewing the manuscript. Sean C. Blackwell: study design, and drafting and reviewing the manuscript.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.

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