Objective: The study aimed to estimate weekly differences in the prevalence of a short cervix during the period of 18+0 to 23+6 weeks of gestation in pregnant women with and without a history of previous preterm delivery (PTD). Design: An observational study was conducted. Methods:Setting and participants: 20,002 pregnant women, 18,591 without a history of previous PTD (low risk) and 1,411 with at least one previous PTD (high risk), were evaluated at 18+0 to 23+6 weeks + days of gestation. Weekly differences in the prevalence of a short cervix (≤25 mm, ≤20 mm, and ≤15 mm) between women with and without previous PTD were estimated. Results: High-risk women had a significantly higher prevalence of a short cervix, defined as either ≤25 mm (4.4% vs. 2.2%; p < 0.0001) or ≤20 mm (2.4% vs. 1.2%; p < 0.0001) but not for ≤15 mm (1.2% vs. 0.9%; p < 0.2) as compared to low-risk pregnant women. The odds ratio for a short cervix ≤25 mm in high-risk as compared to low-risk women was 2.0 (95% CI 1.54–2.61; p < 0.0001). Among low-risk women, those evaluated at 22 or 23 weeks of gestation had a significantly higher prevalence of a short cervix ≤25 mm (3.8% vs. 1.9%; p < 0.0001), ≤20 mm (2.4% vs. 0.98%; p < 0.0001), and ≤15 mm (1.6% vs. 0.7%; p < 0.0001) than low-risk women scanned between 18 and 21 weeks of gestation. Similar results were observed for high-risk women. Limitations: No gestational age at delivery was evaluated. Conclusion: There is higher prevalence of short cervix when pregnant women are evaluated at 22+0 to 23+6 than at 18+0 to 21+6 weeks of gestation.

Universal cervical length (CL) screening is usually performed between 18+0 and 23+6 weeks of gestation to identify women with a short cervix who can be treated with vaginal progesterone to reduce the rate of preterm delivery (PTD) [1‒4]. Different thresholds have been proposed to define a short cervix, either 15 mm [5, 6], 20 mm [7], 25 mm [8, 9], or even 30 mm [10, 11], dependent upon the characteristics of the population.

However, regardless of the threshold, the same CL defines a short cervix throughout 18+0 to 23+6 weeks of gestation [9, 11, 12], based on the concept that CL does not change during this period in pregnancy [13]. Nevertheless, there is a physiologic shortening of the cervix, and a different CL distribution as gestation advances [14] that, in turn, may affect the prevalence of a short cervix if the scan is performed either at 18 weeks or at 24 weeks of gestation [4, 15‒17]. The CL threshold selected to define a short cervix is of major importance as it defines the prevalence of the condition and the number of women considered to be at risk [18] who may benefit from preventive treatment. The aim of this study was to describe weekly differences in the prevalence of a short cervix during the period of 18+0 to 23+6 weeks of gestation between pregnant women considered as low risk and high risk for PTD based on a history of previous PTD.

The present study was conducted in the Maternal-Fetal Medicine (MFM) ultrasound units of the University of Texas McGovern Medical School Department of Obstetrics and Gynecology between September 2017 and March 2020. Pregnant women with a singleton pregnancy were analyzed between 18+0 and 23+6 weeks of gestation. Women at high risk for PTD had a history of at least one previous PTD <37 weeks and/or previous cervix surgery (loop electrosurgical excision procedure).

Ultrasound Examinations

Ultrasound evaluations were performed by Radio Diagnostic Medical Sonographer (RDMS) and Cervical Length Education and Review (CLEAR) certified sonographers and reviewed by an MFM board-certified or board-eligible specialist. The CLEAR program integrates an online course with an examination and image review for transvaginal CL measurement [19, 20]. Ultrasound examinations were performed with the GE Voluson E-8 system and abdominal and transvaginal ultrasound probes.

All patients had a transabdominal scan for detailed evaluation of fetal anatomy and biometry. CL was measured according to the CLEAR requirements for transvaginal ultrasound [19, 20]: empty bladder, identification of the mid-sagittal plane of the cervix, cervical image magnified as occupying at least 50% of the ultrasound screen, clear visualization of the internal cervical os, external cervical os, and endocervical canal, and a similar thickness of the anterior and posterior cervical lips. Calipers were placed from the internal os to the external os by following the endocervical canal [21, 22]. Three measurements were obtained, and the shortest was considered as representative of the patient. A short cervix was defined as a CL ≤25.0 mm, ≤20.0 mm, or ≤15.0 mm.

Analysis

The prevalence of a short cervix (≤25.0 mm, ≤20.0 mm, and ≤15.0 mm) at each week during the period 18+0 and 23+6 weeks was estimated in low- and high-risk pregnant women. Weekly differences within each group and between the two groups were estimated by Kruskal-Wallis and Fisher’s exact test, or a χ2 test with Yates correction; a p value <0.05 was considered significant. Logistic regression analysis was applied to estimate the risk for a short cervix in high-risk pregnant women. Statistical analyses were performed using SPSS and MedCalc statistical software.

A total of 20,002 women were evaluated: 18,591 (92.9%) were considered as low risk and 1,411 (7.1%) as high risk. Clinical characteristics of the two study groups are shown in Table 1. High-risk pregnant women had a significantly higher weight than low-risk pregnant women (p < 0.001). Among high-risk women, 78.5% (1,109//1,411) had one previous PTD, 15.8% (223/1,411) had two, 3.8% (54/1,411) had three, 1.2% (17/1,411) had four, and 0.5% (8/1,411) had five or more previous PTDs.

Table 1.

Maternal characteristics of the study population

 Maternal characteristics of the study population
 Maternal characteristics of the study population

Table 2 and Figure 1 describe the prevalence of a short cervix in both groups, according to the different thresholds at each gestational week between 18+0 and 23+6 weeks of pregnancy. Overall, high-risk women had a significantly higher prevalence of a short cervix, defined either as ≤25 mm (high risk 4.4% [62/1,411] vs. low risk 2.2% [403/18,591]; p < 0.0001) and as ≤20 mm (high risk 2.4% [35/1,411] vs. low risk 1.2% [224/18,591]; p < 0.0001) but not for ≤15 mm (high risk 1.2% [17/1,411] vs. low risk 0.9% [161/18,591]; p < 0.2, Figure 1a).

Table 2.

Prevalence of a short cervix at each week of gestation in the period of 18+0 to 23+6 weeks of gestation in low-risk and high-risk pregnant women

 Prevalence of a short cervix at each week of gestation in the period of 18+0 to 23+6 weeks of gestation in low-risk and high-risk pregnant women
 Prevalence of a short cervix at each week of gestation in the period of 18+0 to 23+6 weeks of gestation in low-risk and high-risk pregnant women
Fig. 1.

a Overall differences in short cervix using different cut-off values to define a short cervix between high- and low-risk pregnant women between 18+0 and 23+6 weeks/days of gestation. Weekly differences in the prevalence of short cervix defined as ≤25 mm (b), ≤20 mm (c), ≤15 mm (d), between 18+0 to 23+6 weeks of gestation in low- and high-risk pregnant women.

Fig. 1.

a Overall differences in short cervix using different cut-off values to define a short cervix between high- and low-risk pregnant women between 18+0 and 23+6 weeks/days of gestation. Weekly differences in the prevalence of short cervix defined as ≤25 mm (b), ≤20 mm (c), ≤15 mm (d), between 18+0 to 23+6 weeks of gestation in low- and high-risk pregnant women.

Close modal

Table 3 shows differences in the prevalence of a short cervix defined as either ≤25 mm, ≤20 mm, or ≤15 mm, when pregnant women were evaluated before or after 22 weeks of gestation. Among low-risk women, those evaluated at 22 or 23 weeks of gestation had a significantly higher prevalence of a short cervix ≤25 mm (3.8% vs. 1.9%; p < 0.0001, Fig. 1b), ≤20 mm (2.4% vs. 0.98%; p < 0.0001, Fig. 1c), and ≤15 mm (1.6% vs. 0.7%; p < 0.0001, Fig. 1d) than low-risk women scanned between 18 and 21 weeks of gestation. Among high-risk women, those evaluated at 22 or 23 weeks of gestation had a significantly higher prevalence of a short cervix ≤20 mm (4.6% vs. 2.0%; p < 0.03, Fig. 1c) and ≤15 mm (2.5% vs. 0.9%; p < 0.05, Fig. 1d), but not for a short cervix ≤25 mm (5.4% vs. 4.1%; p = 0.3, Fig. 1a), than high-risk women scanned between 18 and 21 weeks of gestation. No differences in the prevalence of a short cervix at any CL threshold were observed among low-risk women with a gestation between 18+0 and 21+6 weeks of pregnancy. The relative risk (RR) for a short cervix defined as ≤25 mm in women with a history of previous PTD was 2.0 (95% CI 1.54–2.61; p < 0.0001); for a short cervix ≤20 mm, RR = 2.0 (95% CI 1.43–2.9; p < 0.0001), and for a short cervix ≤15 mm, RR = 1.4 (95% CI 0.83–2.7; p < 0.2).

Table 3.

Differences in the prevalence of a short cervix when a pregnant woman is evaluated either before or after 22 weeks of gestation

 Differences in the prevalence of a short cervix when a pregnant woman is evaluated either before or after 22 weeks of gestation
 Differences in the prevalence of a short cervix when a pregnant woman is evaluated either before or after 22 weeks of gestation

Our results show that among high-risk and low-risk pregnant women, the prevalence of a short cervix, defined as either ≤25 mm, ≤20 mm, or ≤15 mm at 22+0 to 23+6 weeks of gestation, is approximately two times higher than at 18+0 to 21+6 weeks of gestation. If CL is evaluated between 18+0 and 21+6 weeks of gestation, one-half of the group of women with a short cervix will not be identified as when the scan is performed between 22+0 and 23+6 weeks of gestation.

Our low-risk population showed a prevalence of CL ≤15 mm (0.9%) and CL ≤20 mm (1.2%) lower than those reported by Fonseca et al. [23] (CL ≤15 mm, 1.7%) and by Hassan et al. [7] (CL ≤25 mm, 2.3%), respectively. Such prevalences reported by Fonseca and Hassan were similar to those observed in our high-risk population (CL <15 mm [1.2%] and CL <20 [2.4%]). Nevertheless, and despite these differences and similarities, our results suggest that the distribution of CL values changes during the 18+0- to 23+6-week period, and that the measurement of CL either at the beginning or at the end of this period has a significant impact in the characterization of low-risk and high-risk pregnancies and in the number of women eligible for preventive treatment. We observed significant differences in the prevalence of short cervix ≤25 and ≤20 mm but not ≤15 mm between high-risk and low-risk pregnant women. The lack of statistical difference in the prevalence of short cervix ≤15 mm is most probably due to the reduced number of women with CL <15 mm in the high-risk group (n = 17). There is indeed a higher frequency of CL <15 mm (1.2%) in high-risk women as compared to low-risk women (0.9%). By increasing the sample size in the high-risk group, this difference may become significant.

Evaluating women during 18+0 to 21+6 weeks of gestation will reduce the number of women considered to be at risk for preterm birth while demonstrating a potential advantage of starting vaginal progesterone treatment earlier. On the other hand, evaluating women at 22+0 to 23+6 weeks of gestation will increase by two-fold the number of women considered to be at risk for preterm birth, with more women receiving vaginal progesterone, but with the disadvantage of starting the treatment later in pregnancy. The clinical and economic impact of these variations in the number of women considered at risk who receive treatment and on the effect of progesterone started at different times within the period of 18+0 to 23+6 weeks has not been evaluated. Based on screening in the “mid-trimester” performed from 16 weeks of gestation until 25 weeks [17, 24], variations in the prevalence of a short cervix may be, at least partially, responsible for differences in the reported frequencies of a short cervix among several groups of pregnant women.

Two main advantages of a fixed CL value that defines a short cervix are as follows: (1) it is easy to remember, and (2) it can be used to standardize clinical strategies, i.e., treatment or no treatment below or above a specific value. However, this approach does not always perform similarly in all cases, as populations may have different characteristics. One alternative is the use of customized reference charts for CL to define a short cervix to be below a defined percentile [25]. This approach will include a similar fraction of patients considered at risk independently of the study population or gestational age at evaluation, and it has the advantage of comparing CL measurements to the distribution obtained from the targeted population.

Nevertheless, an alternative explanation of a higher prevalence of a short cervix at 22+0 to 23+6 weeks is that indeed these women have a higher risk of PTD, and shortening of the cervix will be more apparent after 22 weeks than at earlier gestational weeks – thus, the closer the time of cervical evaluation to the obstetric event, the higher the probability of finding a short cervix. Among pregnant women with a short cervix at 18 weeks of gestation, even at a lower number, some, in fact, may have a higher probability to be pathologic and may benefit by an earlier start of treatment with vaginal progesterone. The clinical impact of starting vaginal progesterone at the beginning of the 18+0-week gestational period should be evaluated, as these women would have the advantage of at least 4 to 5 additional weeks of preventive treatment than those identified after 22 weeks of gestation. The lack of knowledge concerning the differences in the prevalence of a short cervix during the 18+0- to 23+6-week period, and on the effect of progesterone when started at 18+0 weeks or at 23+6 weeks/days, limits general assumptions on the performance of a short cervix during the mid-trimester scan that can identify pregnant women at risk for PTD and prevent the adverse outcome of PTD.

Strengths and Weaknesses

Our study included a high number of low-risk and high-risk patients per gestational week evaluated by trained sonographers following standard protocols. Analyses were performed by using different cut-off values to define a short cervix. The lack of documentation of gestational age at delivery and prevalence of PTD were limitations of this study.

Low- and high-risk pregnant women evaluated at 22+0 to 23+6 weeks of gestation have a two-fold increase in prevalence of a short cervix than women evaluated at 18+0 to 21+0 weeks of gestation.

We would like to thank all RDMS sonographers who contributed in the implementation and establishment of the universal transvaginal CL-screening program in the University of Texas McGovern Medical School Department of Obstetrics and Gynecology.

A waiver was granted for retrospective chart review by the Institutional Review Board at the McGovern Medical School at the University of Texas Health Science Center at Houston (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.

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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