The Institute for Advanced Medical Education is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians.

The Institute for Advanced Medical Education designates this educational activity for a maximum of 1 AMA PRA Category 1 Credit(s) TM. Physicians should only claim credit commensurate with the extent of their participation in the activity.

These credits are accepted by the American Registry for Diagnostic Medical Sonography (ARDMS).

Faculty:
Lyndon M. Hill, MD
Professor Obstetrics and Gynecology
Medical Director Ultrasound
Magee Women's Hospital
Pittsburgh, PA

Course: Cervical Length In Preterm Labor Prediction

Target Audience: Physicians, sonographers and others who perform and/or interpret obstetrical ultrasound.

System requirements: In order to complete this program you must have a computer with a recent version of Internet Explorer or Netscape, and a printer, which is configured to print from the browser.

For any questions or problems concerning this program or for problems related to the printing of the certificate please contact IAME at (914) 921-5700 or email us.

Estimated Time for Completion of tutorial: One hour
Date of Release and Review: April 24, 2006
Expiration Date: April 24, 2009

Disclosure: In compliance with the Essentials and Standards of the ACCME, the author of this CME tutorial is required to disclose any significant financial or other relationships they may have with the manufacturer(s) of any commercial product(s) or provider(s) of any commercial service(s) discussed in this program.

Dr. Lyndon Hill discloses no relevant financial relationships with commercial interests.

IAME discloses no relevant financial relationships with commercial interests. 

IAME Statement on Privacy and Confidentiality

Cervical Length In Preterm Labor Prediction
Lyndon M. Hill, MD
Professor Obstetrics and Gynecology
Medical Director Ultrasound
Magee Women's Hospital
Pittsburgh, PA

(DIRECT TO QUIZ)

Objectives
After completing this course, the participant should be able:

• Describe when and how to evaluate cervical length.
• Identify patients at risk for preterm labor and the relationship of cervical length to preterm labor.
• Apply multifactorial assessment of patients at risk for preterm delivery to improve positive predictive value.

Introduction
Prematurity causes 70% of fetal/neonatal deaths. 11.4% of births are at < 37 weeks' gestation. The rate of prematurity increases dramatically with the number of fetuses - singletons 10%; twins 54.9%; and triplets 93.6% 1 .

Methods to assess cervical lengths have gradually evolved over the past decade. The expansion of the bladder that is required with the transabdominal approach results in cervical lengthening (Fig. 1). The translabial or transperineal technique (Fig. 2) is not always successful because of reduced visualization secondary to bowel gas 2 . The transvaginal approach has, therefore, become the accepted method for cervical length assessment.

Figure 1 - Transabdominal ultrasound examination of the membranes hourglassing through the cervix (x. . .x) into the vagina (+. . .+).		Figure 2 - Translabial cervical length of 38 mm (markers).
To view an enlargement, click on the image.

Cervical length is inversely related to the risk of preterm labor 3 . The specific reason or reasons for this association have yet to be defined. Uterine contractions and the inherent tensile strength of the cervix undoubtedly play a role. Bacterial vaginosis is a known risk factor for preterm delivery. Hence, the cervix may also act as a barrier between the vagina and amniotic cavity 4 .

In women with a diagnosis of preterm labor, the transvaginal examination of the cervix has been found to be superior to a digital exam in predicting preterm delivery 6 .

Rozenberg et al 7 have stopped utilizing digital exams in their unit on patients with symptoms of preterm labor. A cervical length < 26 mm with at least 2 contractions in 10 minutes before 34 weeks' gestation are used as criteria for admission and treatment.

Transvaginal Cervical Examination
The patient's bladder should be emptied prior to the exam. A small amount of gel is placed on the previously sterilized transducer and then it is covered with a condom. The patient is asked to insert the transducer. If she is unable or unwilling, the sonographer can slowly insert the probe. He/she should stop if there is too much resistance or patient discomfort. A sagittal image of the cervix is obtained. The transducer is slowly removed until the image begins to blur. The transducer is then reinserted until the image is clear. The cervix should occupy at least 50% to 75% of the screen. The cervix is measured from the internal to the external cervical os (Fig. 3). The cervical glands (Fig. 4) can help determine the length of the endocervical canal. If the cervix is not straight, a curved distance measurement (Fig. 5) or two end-to-end straight measurements must be used to obtain an accurate cervical length. The cervical canal should be equidistant from the anterior and posterior cervical walls (Fig. 1). At least 3 measurements are obtained; the shortest, best measurement is recorded. When the above steps are followed Burger et al 8 reported an average intraobserver difference of 1.24 mm; 87% of measurements were within 2 mm.

Figure 3		Figure 4		Figure 5
Transvaginal cervical length measurement from the internal to external cervical os (arrows).		Transvaginal ultrasound of the endocervical glands (arrows).		Curved distance measurement of cervical length
To view an enlargement, click on the image.

A dynamically changing cervix has been associated with uterine contractions. In term women in labor, the cervix shortens approximately 50% during a contraction 9 . Some authors have recommended fundal pressure in an attempt to mimic this effect on the cervix. The presence of induced cervical shortening has been found in some studies to increase the positive predictive value of the cervical measurement in diagnosing preterm labor. Other authors have not found this to be the case 10 .

In order to evaluate the cervix for the dynamic changes that occurs in 50% of women with preterm labor, a 10-minute observation period is required 11 . A dynamically changing cervix, either spontaneously, or after fundal pressure, improves the predictive accuracy of cervical length in predicting preterm delivery 12 .

Focal myometrial contractions may obscure the internal cervical os and result in a longer measurement (Fig. 6).

Figure 6 - Lower uterine segment contractions (c) resulting in an erroneously longer cervical length. To view an enlargement, click on the image.

In most studies the addition of cervical funneling does not improve the predictive accuracy of cervical length in preterm labor prediction (Fig. 7) 13 . This may, in part, be due to the wide variations noted in funnel measurement. Rust et al 14 have found that as a categorical variable (present or absent), a funnel is a significant risk factor for preterm labor. However, the latter study had a small sample size and was retrospective in nature. Additional prospective studies will be required to determine if the presence of a funnel adds to the predictive accuracy of cervical length in predicting preterm delivery.

Figure 7 - There is a cervical funnel with a length (x. . .x) of 1.50 cm and a width (+. . .+) of 0.84 cm. The remaining cervix (?. . .?) measures 0.90 cm. To view an enlargement, click on the image.

Cervical length is a continuous variable, with a mean of 35 mm to 40 mm. A cervical length of 25 mm has generally been found to be at the 10 th percentile 3,15 .

For patients at highest risk for preterm delivery (prior 24 week delivery, 2nd trimester losses), an initial examination at 15 to 16 weeks should be considered. For patients at a lower risk (cone biopsy, uterine malformations), a first exam could be obtained during the 18 to 20 weeks anatomy scan.

Cervical length and fetal fibronectin may identify different patients at risk for preterm labor. The predictive accuracy of either test is based on the pretest probability of preterm labor in the population under investigation 20 .

Honest et al 18 conducted a meta-analysis of 40 studies in women with symptoms of preterm labor and of 28 studies of women without symptoms of labor. In the asymptomatic and symptomatic groups a positive fibronectin had a likelihood ratio (LR) of 4.01 and 5.42, respectively. The LR of a negative fibronectin was 0.78 and 0.25 for the asymptomatic and symptomatic women, respectively. Fetal fibronectin is most accurate in predicting preterm labor within 7 to 10 days among women with symptoms of preterm labor.

532 pregnancies from 3 studies presented with preterm labor at 24-36 weeks' gestation. If the cervical length was < 15 mm, delivery occurred within 3 days 21 .

As a patient's number of preterm births increases and the gestational age at preterm delivery decreases, the likelihood of another preterm delivery rises by 3 to 4-fold. Yost and co-workers 22 found that these historical variables did not affect the predictive accuracy of a cervical length < 25 mm between 16 and 19 weeks' gestation. However, because of the small sample size, the power of the study to detect a clinically significant difference was low. Until additional studies are reported, prior preterm delivery should be considered a categorical variable that increases the risk of preterm labor.

A cervical length <= 25 mm has a 13-fold risk for preterm delivery in women with uterine anomalies. In one study of 64 pregnant women with uterine anomalies, the sensitivity, specificity, positive and negative predictive values of a cervical length <= 25 mm for preterm delivery were 71%, 91%, 50%, and 96%, respectively 23 . The positive predictive value is higher if an abnormally shortened cervix was detected at 14 to 18 weeks in contrast to 18 to 22 weeks 24 . In a blinded study of patients with a history of preterm deliveries, a shortened cervical length had a sensitivity and positive predictive value of 69% and 55%, respectively in predicting preterm labor 12 . Hence, patients at highest risk may be evaluated between 14 and 18 weeks' gestation, while those at slightly lower risk need not be evaluated until 18 to 22 weeks 25 .

Faster rates of cervical shortening between 16 and 24 weeks' gestation is an additional independent factor associated with an increased prevalence of preterm labor 26 . The rate of cervical shortening in cases of incompetent cervix diagnosed between 15 and 19 weeks' gestation is - 0.52 cm/week. Between 15 and 24 weeks' gestation the length of a competent cervix is essentially unchanged 26 .

Chorioamniotic membrane visualization at the internal cervical os indicates disruption of the decidua-membrane interface and is an independent risk factor for preterm delivery (Fig. 8). Finally, minimal (1-4 mm) endocervical canal dilatation has been associated with an increased risk of preterm labor (Fig. 9) 27 .

Figure 8 - A cervical funnel extends to the external cervical os. There is a separation of the decidua membrane interface (arrow). Contractions (c) have narrowed the cervical canal. Blood (b) is present within the lower aspect of the amniotic sac.		Figure 9 - Minimal endocervical canal dilatation that has been associated with an increased risk of preterm labor.
To view an enlargement, click on the image.

Short cervix and intrauterine infection
Patients with a cervical length < 15 mm have a higher rate of positive amniotic fluid cultures than patients with a cervical length > 15 mm (26.3% versus 3.8%) 28 . The earlier the gestational age and the shorter the cervix, the higher the likelihood of intra-amniotic infection 29 . This relationship may be due to: 1) intra-amniotic infection resulting in contractions and a shortened cervix; or 2) a short cervix may predispose to ascending infection 28 .

Twins
Cervical length <=25 mm in twin pregnancies at 27 weeks' gestation predicts delivery before 34 weeks (sensitivity 77%, specificity 86%, positive predictive value 34%) better than a digital examination 30 .

Cervical length <= 25 mm at 18 weeks (likelihood ratio + 9.7, sensitivity 14.3%) and < 22 mm at 24 weeks (likelihood ratio + 9.6, sensitivity 28.6%) were found to be the best predictors of preterm labor in twins. The poor sensitivity of cervical length as a predictor of preterm labor in twins makes it unsuitable as a single test.

The negative predictive value of a long cervix has also been evaluated. It has been suggested that cervical length > 35 mm at 24-26 weeks' gestation predicts that a twin pregnancy will deliver at term (negative predictive value 86% - 97%) 31,32 . However, the likelihood ratio of 0.54 to 0.80 indicates that this is not a valuable predictor of delivery >= 35 weeks 31,32 .

3D Cervical Assessment
The measurement of cervical length with 3D sonography (Fig. 10a, b) is longer than with the 2-dimensional approach. Bega et al 33 found a > 5 mm difference between the 2 measurements. The longer the cervix, the greater the difference between the 2D and the 3D approach 34 .

Three dimensional measurement of cervical length:
Figure 10a - a multiplanar display of the cervix. The image was acquired in a sagittal plane (a); b = transverse; c = coronal		Figure 10b - the reconstructed image of the cervix provides a reliable measurement of length
To view an enlargement, click on the image.

Severi et al 35 also found a significant difference (p < 0.001) between cervical length measured with 2D ultrasound (37.6 mm) and 3D ultrasound (39.8 mm). A correct, mid-sagittal plane was more accurately determined with 3D in contrast to 2D ultrasound 35 . Whether the difference in cervical length between the 2 measurement techniques is clinically significant remains to be determined.

Cervical length generally increases after a cerclage. The lengthening of the cervix after cerclage is associated with a higher likelihood of a term delivery 37,38 . A cervical length < 25 mm or a cervical length of < 10 mm above the suture have been found to be predictive of preterm labor (Fig. 11) 38 .

Figure 11 - Cervical cerclage (arrows). The cervix above the cerclage measures 5 mm. The cervix from the cerclage to external cervical os measures 18 mm. To view an enlargement, click on the image.

In a low-risk population, cervical length is not an effective screening tool 3 . Because of the low prevalence of disease, the sensitivity of cervical length is decreased and the false positive rate is increased. In order to improve the positive predictive value of a cervical length measurement in low-risk women, a length of 15 mm (0.6% 39 - 1.6% 40 ) rather than 25 mm should be considered too short.

Prior spontaneous preterm birth, fetal fibronectin, cervical length, and bacterial vaginosis are associated with early preterm birth. The association of the first 3 with preterm delivery is more than additive 41 . These associations indicate that there is more than one pathway resulting in preterm delivery. In the future, a multifactorial assessment of patients at risk for preterm delivery should improve the positive predictive value of our testing schema. Specific therapeutic regimens depending upon the etiology or etiologies of preterm labor will be required and should improve our current limited successfully inhibition of preterm labor.

References

1. Ventura SJ, Martin JA, Curtain SC , Mathews TJ. Births: Final Data for 1997. Nat Vital Stat Report 1999;47(18):1-96.
2. Cicero S, Skentou C, Souka A et al. Cervical length at 22-24 weeks of gestation: comparison of transvaginal and transperineal-translabial ultrasonography. Ultrasound Obstet Gynecol 2001;17:335-340.
3. Iams JD, Goldenberg RL, Meis PJ, Mercer BM, Moawad A 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. N Engl J Med 1996;334:567-572.
4. Edwards RK, Ferguson RJ, Shuster JJ, Theriaque D, Gentry S et al. Assessment of cervical antibody concentrations fails to enhance the value of cervical length as a predictor of preterm delivery. Am J Obstet Gynecol 2005;192:887-891.
5. Phelps JY, Higby K, Smyth MW, Ward JA, Arredondo F et al. Acuracy and intraobserver variability of simulated cervical dilatation measurements. Am J Obstet Gynecol 1995;173:942-945.
6. Gomez R, Galasso M, Romero R, Mazor M, Sorukin Y et al. Ultrasonographic examination of the uterine cervix is better than cervical digital examination as a predictor of the likelihood of premature delivery in patients with preterm labor and intact membranes. Am J Obstet Gynecol 1994;171:956-965.
7. Rozenberg P, Rudant J, Chevret S, Boulogne AI. Repeat measurement of cervical length after successful tocolysis. Obstet Gynecol 2004;104:995-999.
8. Burger M, Weber-Rössler TW, Willmann M. Measurement of the pregnant cervix by transvaginal sonography: an interobserver study and new standards to improve the interobserver variability. Ultrasound Obstet Gynecol 1997;9:188-193.
9. Saito M, Kozuma S, Kikuchi A, Sakai M, Fujii T et al. Sonographic assessment of the cervix before, during and after a uterine contraction is effective in predicting the course of labor. Ultrasound Obstet Gynecol 2003;22:604-608.
10. Berghella V, Bega G, Tolosa JE, Berghela M. Ultrasound assessment of the cervix. Clin Obstet Gynecol 2003;46:947-962.
11. Kurtzman JT, Jenkins SM, Brewster WR. Dynamic cervical changes during real-time ultrasound: prospective characterization and comparison in patients with and without symptoms of preterm labor. Ultrasound Obstet Gynecol 2004;23:574-578.
12. Owen J, Yost N, Berghella V, Thom E, Swain GA et al. Mid-trimester endovaginal sonography in women at high risk for spontaneous preterm birth. JAMA 2001;286:1340-1348.
13. Owen J, Yost N, Berghella V, MacPherson C, Swain M et al. Can shortened mid-trimester cervical length predict very early spontaneous preterm birth? Am J Obstet Gynecol 2004;191:298-303.
14. Rust OA, Atlas RO, Kimmel S, Roberts WE, Hess LW. Does the presence of a funnel increase the risk of adverse perinatal outcome in a patient with a short cervix? Am J Obstet Gynecol 2005;192:1060-1066.
15. Anderson HF, Nugent CE, Wanty SO, Hayashi RH. Prediction of risk for preterm delivery by ultrasonographic measurement of cervical length. Am J Obstet Gyecol 1990;163:859-867.
16. Berghella V, Tallucci M, Desai A. Does transvaginal sonographic measurement of cervical length before 14 weeks predict preterm delivery in high-risk pregnancies? Ultrasound Obstet Gynecol 2003;21:140-144.
17. Heath VCF, Southall TR, Souka AP, Elisseou A, Nicolaides KH. Cervical length at 23 weeks of gestation: prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol 1998;12:312-317.
18. Honest H, Bachman LM, Gupta JK, Kleijnen J, Khan KS . Accuracy of cervicovaginal fetal fibronectin test in predicting risk of spontaneous preterm birth: systematic review. BMJ 2002;325:301-320.
19. Peaceman AM, Andrews WW, Thorp JM, Cliven SP, Lukes A et al. Fetal fibronectin as a predictor of preterm birth in patients with symptoms: a multicenter trial. Am J Obstet Gynecol 1997;177:13-18.
20. Goldenberg RL, Iams JD, Das A, Mercer BM, Meis PJ et al. The Preterm Prediction Study: sequential cervical length and fetal fibronectin testing for the prediction of spontaneous preterm birth. Am J Obstet Gynecol 2000;182:636-643.
21. Tsoi E, Fuchs IB, Rane S, Geents L, Nicolaides KH. Sonographic measurement of cervical length in threatened preterm labor in singleton pregnancies with intact membranes. Ultrasound Obstet Gynecol 2005;25:353-356.
22. Yost NP, Owen J, Berghella V, MacPherson C, Swain M et al. Number and gestational age of prior preterm births does not modify the predictive value of a short cervix. Am J Obstet Gynecol 2004;191:241-246.
23. Airoldi J, Berghella V, Sehdeu H, Ludmin J. Transvaginal ultrasonography of the cervix to predict preterm birth in women with uterine anomalies. Obstet Gynecol 2005;106:553-556.
24. 24. Odibo AO, Talucci M, Berghella V. Prediction of preterm premature rupture of the membranes by transvaginal ultrasound features and risk factors in a high-risk population. Ultrasound Obstet Gynecol 2002;20:245-251.
25. 25. Berghella V, Tolosa JE, Kuhlman KA, Weine S, Bolagnese RJ et al. Cervical ultrasonography compared to manual examination as a predictor of preterm delivery. Am J Obstet Gynecol 1997;177:723-730.
26. Guzman ER, Mellon C, Vintzileos AM, Ananth CV, Walters C et al. Longitudinal assessment of endocervical canal length between 15 and 24 weeks' gestation in women at risk for pregnancy loss or preterm birth. Obstet Gynecol 1998;92:31-37.
27. Yost N for the NICHD MFMU Nework: Mid-trimester vaginal sonography: Ancillary markers for spontaneous preterm birth. Am J Obstet Gynecol 2001;185:(suppl)144.
28. Gomez R, Romero R, Nien JK, Chaiworapongsa T, Medina L et al. A short cervix in women with preterm labor and intact membranes: A risk factor for microbial invasion of the amniotic cavity. Am J Obstet Gynecol 2005;192:678-689.
29. Watts DH, Krohn MA, Hillier SL, Eschenbach DA. The association of occult amniotic fluid infection with gestational age and neonatal outcome among women in preterm labor. Obstet Gynecol 1992;79:351-357.
30. Vayssiére C, Favre R, Audibert F, Chauvet MP, Gaucherand P et al. Cervical assessment at 22 and 27 weeks for the prediction of spontaneous birth before 34 weeks in twin pregnancies: is transvaginal sonography more accurate than digital examination? Ultrasound Obstet Gynecol 2005;26:707-712.
31. Gibson JL, Macara LM, Owen P, Young D, Macauley J et al. Prediction of preterm delivery in twin pregnancy: a prospective, observational study of cervical length and fetal fibronectin testing. Ultrasound Obstet Gynecol 2004;23:561-566.
32. Imseis HM, Albert A, Iams JD. Identifying twin gestations at low risk for preterm birth with a transvaginal ultrasonographic cervical measurement at 24 to 26 weeks' gestation. Am J Obstet Gynecol 1997;177:1149-1155.
33. Bega G, Lev-Toaff A, Khulman K et al. Three-dimensional multiplanar transvaginal ultrasound of the cervix in pregnancy. Ultrasound Obstet Gynecol 2000;16:351-358.
34. Towner D, Boe N, Lou K, Gilbert WM. Cervical length measurements in pregnancy are longer when measured with three-dimensional transvaginal ultrasound. J Maternal Fetal Neonat Med 2004;16:167-170.
35. Severi FM, Bocchi C, Florio P, Picciolini E, D'Aniello G et al. Comparison of two-dimensional and three-dimensional ultrasound in the assessment of the cervix to predict preterm delivery. Ultrasound Med Biol 2003;29:1261-1265.
36. Berghella V, Odibo AO, To MS, Rust OA, Althuisius SM. Cerclage for short cervix on ultrasonography: meta-analysis of trials using individual patient-level data. Obstet Gynecol 2005;106:181-189.
37. Althuisius SM, Dekker GA , van Geijn HP, Hummel P. The effect of therapeutic McDonald cerclage on cervical length as assessment by transvaginal ultrasonography. Am J Obstet Gynecol 1999;180:366-370.
38. Guzman ER, Houlihan C, Vintzileos A, Ivan J, Berito C et al. The significance of transvaginal ultrasonographic evaluation of the cervix in women treated with emergency cerclage. Am J Obstet Gynecol 1996;175:471-476.
39. Hassan SS, Romero R, berry SM, Dang K, Blackwell SC et al . Patients with an ultrasonographic cervical length £ 15 mm have nearly a 50% risk of early spontaneous preterm delivery. Am J Obstet Gynecol 2000;182:1458-1467.
40. To MS, Skentou C, Liao AW, Cacho A, Nicolaides KH. Cervical length and funneling at 23 weeks of gestation in the prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol 2001;18:200-203.
41. Goldenberg RL, Iams JD, Mercer BM, Meis PJ, Moawad AH et al. The preterm prediction study: the value of new vs. standard risk factors in predicting early and all spontaneous preterm births. Am J Public Health 1998;88:233-238.

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