How to measure cervical length
2015-01-29

How to... Practical advice on imaging-based techniques and investigations with accompanying slides and videoclips online Cervical-length measurement using transvaginal sonography (TVS) is an essential part of assessing the risk of preterm delivery. At mid-gestation, it provides a useful method with which to predict the likelihood of subsequent preterm birth in asymptomatic women. In women who present with threatened spontaneous preterm labor, TVS measurement of cervical length can help to distinguish between 'true' and 'false' spontaneous preterm labor. Additionally, there is some evidence that measurement of the cervix at the 11+0 to 13+6-week scan can help to establish the risk of preterm birth1, 2. To et al.3 reported on cervical-length measurement between 22 and 24 weeks' gestation in 39 000 women with a singleton pregnancy. The cervical length was found to be distributed normally, with a mean length of 36 mm. In about 1% of the women, the length was 15 mm or less. This cut-off is generally used to define the high-risk group in interventional studies4. In most studies focusing on asymptomatic twin pregnancies at 20 to 24 weeks, a cut-off of 25 mm is applied5. Celik et al.6 used cervical-length measurements obtained between 20 and 24 weeks' gestation, along with maternal history, in more than 58 000 women to create computed risk models for preterm delivery. They compared patients who delivered before 28 weeks, between 28 and 30 weeks, between 31 and 33 weeks, and between 34 and 36 weeks' gestation. For a 10% false-positive rate, the sensitivities were 81%, 59%, 53% and 29%, respectively. In a Health Technology Assessment report, Honest et al.7 summarized the results of five studies that used cervical-length measurements between 20 and 24 weeks, with cut-offs of 20–30 mm, to predict preterm birth before 34 weeks' gestation. The resultant positive likelihood ratios ranged from 2.3 for 30 mm to 7.6 for 20 mm. It has been shown that only 15% of symptomatic patients presenting with preterm contractions will actually deliver within the next 7 days8. Cervical length can be used to distinguish between 'true' and 'false' labor. Sotiriadis et al.9 summarized six studies, including about 1800 women, that assessed the risk for preterm delivery within 1 week, based on cervical length at the time of admission. With a cut-off of 15 mm, the sensitivity was about 60% for a false-positive rate of about 10%. The respective positive and negative likelihood ratios were 5.7 and 0.5. With cut-offs of 20 and 25 mm, the sensitivities increased to 75% and 80%, but at the expense of false-positive rates of 20% and 30%, respectively. Recent work10 has shown that cervical length at 11–13 weeks' gestation, in combination with maternal history, can detect about half of the pregnancies that result in preterm delivery before 34 weeks, for a false-positive rate of 10%. However, the difference between the median cervical length of the normal group and that of the preterm-birth group was only 5 mm (32.5 mm vs 27.5 mm)10, which raises doubts as to whether this technique can be used for general screening. There are essentially four methods that can be used to evaluate the uterine cervix: digital examination, transabdominal ultrasound, transperineal ultrasound (TPS) and TVS. It is the digital examination that provides the most comprehensive evaluation of the cervix, assessing dilatation, position, consistency and length. However, this examination suffers from being subjective. It is limited especially in its ability to establish accurately the cervical length. It also cannot detect reproducibly any changes at the internal cervical os and the upper portion of the cervical canal. Ultrasound, with its ability to penetrate the cervical tissue and display its anatomy, makes an ideal modality with which to address both of these issues. Both TVS and TPS are performed with the patient in a dorsal lithotomy position. In order for the measurement to be accurate and reproducible, the following factors need to be taken into account (Figure 1 and Videoclip S1): The patient having a full bladder can increase artificially the cervical length. To et al.11 showed that the mean difference between cervical length measured when the patient's bladder is empty and that measured when the bladder is full is about 4 mm. In addition, a full bladder can obscure the presence of cervical funneling by compressing the two halves of the funnel together (Figure 2). The cervix should be measured along its longitudinal axis, which may be different from the patient's longitudinal axis. The cervical canal, in most cases, is a fairly thin line. It may have a thin layer of hypoechoic contents. This is true especially in the third trimester. In all likelihood, it represents accumulation of mucus, and needs to be differentiated from a thin cervical funnel. This is best done by delineating the course of the fetal membranes: if they are not prolapsing into the cervical canal and are located at the level of the internal cervical os, the presence of a true funnel is unlikely (Figure 3). In order to avoid inclusion of the isthmus into the cervical-length measurement, care must be taken to identify the internal, as well as the external, cervical ora. The external os is identified as the point at which the anterior and posterior lips of the cervix come together. Identification of the location of the internal os can be more challenging. In order to do so, the cervical mucosa must be identified. This is usually hypoechoic, with respect to the surrounding stroma, though occasionally it can be slightly hyperechoic. A thin line of demarcation between the stroma and the cervical mucosa can generally be identified on ultrasound. The point at which the cervical mucosa ends is considered to be the internal cervical os. Of note, the mucosa of the lower uterine segment is much thinner than is the cervical mucosa and is usually difficult to identify on ultrasound (Figure 4). The image should be magnified sufficiently so that the morphology of the cervix is easily identifiable. The cervix should occupy approximately 50–75% of the image. If excessive pressure is applied on the cervix by the probe, the cervix appears artificially to be longer and the presence of a funnel will be obscured. Generally, in order to identify the relevant structures, it is helpful to put some pressure on the cervix initially. However, subsequently this should be reduced to the minimum that is required in order to view the cervix adequately (Videoclip S2). The cervix is not a static structure and the length can vary, for example due to uterine contractions or to different positions of the patient (Videoclip S3). Therefore, sufficient time should be allowed for the examination to detect these changes. It may be helpful to press manually on the uterus or to ask the patient to push downwards to assess the cervical stability. Some even suggest that during the examination the patient stands, with the ultrasound probe inside the vagina. Several (at least three) measurements should be obtained during the course of the examination and the shortest measurement should be used for counseling. Cervical measurement is obtained by placing calipers at the external and the internal ora (Figure 1). In 95% of cases, the difference between two measurements obtained by the same sonographer or by two different sonographers is about 4 mm12. If there is funneling, the caliper should be placed at the apex of the funnel. If the cervix exceeds 25 mm in length, it will be curved in more than 50% of cases13. The standard method of measurement, using a straight line between the internal and the external ora, will underestimate the cervical length in these cases. However, this is of little clinical significance as these patients are at low risk regardless of the exact measurement. In the high-risk group of patients with a cervical length < 16 mm, the cervix will always be a straight line. In the first trimester, the difference in cervical length between patients who are at risk for preterm birth and those who are not is extremely small; therefore, it would seem appropriate to correct for cervical curvature by obtaining the measurement either in segments or by tracing the canal14. In general, the cervix should be assessed by TVS. In cases in which this should be avoided, such as in those with preterm prelabor rupture of membranes, the cervical length can be measured by TPS: the transducer is placed on the perineum and rotated until the complete cervical canal and the internal and external ora can be identified. The placement of the probe is further away from the cervix than it is on TVS; therefore, there is a reduction in the detail in which the cervix is seen. In 95% of cases, the difference between TVS and TPS measurements is within ± 5 mm21. Assessment of cervical length by TAS can be used as an initial evaluation but it should be borne in mind that, especially in cases with a short cervix, this modality tends to overestimate the true cervical length. Therefore, a proper risk assessment should be based on a TVS or TPS measurement (Figure 8). Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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