Clinical Review

The short cervix in pregnancy: Which therapy reduces preterm birth?

OBG Manag. 2003 August;15(8):28-38

Unfortunately, the literature on optimal management is full of conflicting reports. Here, the authors analyze the best data available and offer a detailed management algorithm.

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References

1. Iams JD. Cervical incompetence. In: Creasy RK, Resnik R, eds. Maternal-Fetal Medicine. 4th ed. Philadelphia, Pa: WB Saunders; 1999;445-464.

2. Guzman ER. Cervical length and spontaneous prematurity: laying the foundation for future interventional randomized trials for the short cervix. Ultrasound Obstet Gynecol. 2001;18:195-199.

3. Guzman ER, Forster JK, Vintzileos AM, et al. Pregnancy outcomes in women treated with elective versus ultrasound-indicated cervical cerclage. Ultrasound Obstet Gynecol. 1998;12:323-327.

4. Berghella V, Hass S, Chervoneva I, Hyslop T. Patients with prior second-trimester loss: prophylactic cerclage or serial transvaginal sonograms? Am J Obstet Gynecol. 2002;187:747-751.

5. Rust OA, Atlas RO, Jones KJ, Benham BN, Balducci J. A randomized trial of cerclage versus no cerclage among patients with ultrasonographically detected secondtrimester preterm dilatation of the internal os. Am J Obstet Gynecol. 2000;183:830-835.

6. Rust OA, Atlas RO, Reed J, van Gaalen J, Balducci J. Revisiting the short cervix detected by transvaginal ultrasound in the second trimester: why cerclage therapy may not help. Am J Obstet Gynecol. 2001;185:1098-1105.

7. Rust OA, Atlas RO, Fischl S, Depuy A, Kimmel S, Hess LW. Does cerclage therapy improve perinatal outcome in patients with a history of previous preterm birth and cervical changes on 2nd trimester transvaginal ultrasound? Am J Obstet Gynecol. 2002;187(suppl):S58.-

8. Iams JD, Johnson FF, Sonek K, et al. Cervical competence as a continuum: a study of ultrasonographic cervical length and obstetric performance. Am J Obstet Gynecol. 1995;174:1097-1106.

9. Hassan SS, Romero R, Berry SM, 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.

10. Iams JD, Goldenberg RL, Meis PJ, et al. The length of the cervix and the risk of spontaneous premature delivery. N Engl J Med. 1996;334:567-572.

11. Taipale P, Hilesmaa V. Sonographic measurement of uterine cervix at 18-22 weeks’ gestation and the risk of preterm delivery. Obstet Gynecol. 1998;92:902-907.

12. Hibbard JU, Tart M, Moawad AH. Cervical length at 16-22 weeks’ gestation and risk for preterm delivery. Obstet Gynecol. 2000;96:972-978.

13. Anderson HF, Nugent CE, Wanty SD, Hayashi RH. Prediction of risk for preterm delivery by ultrasonographic measurement of cervical length. Am J Obstet Gynecol. 1990;163:859-867.

14. Heath VC, Southall TR, Souka AP, Elisseou A, Nicholaides KH. Cervical length at 23 weeks of gestation: prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol. 1998;12(5):312-317.

15. Berghella V, Daly SF, Tolosa JE, et al. Prediction of preterm delivery with transvaginal ultrasonography of the cervix in patients with high-risk pregnancies: does cerclage prevent prematurity? Am J Obstet Gynecol. 1999;181:809-815.

16. Berghella V, Tolasa JE, Kuhlman K. Cervical ultrasound compared with manual examination as a predictor of preterm delivery. Am J Obstet Gynecol. 1997;177:723-730.

17. Iams JD, Goldenberg RL, Mercer BM, et al. The preterm prediction study: can low-risk women destined for spontaneous preterm birth be identified? Am J Obstet Gynecol. 2001;184:652-655.

18. Heath VC, Daskalakis G, Zagaliki A, Carvalho M, Nicolaides KH. Cervicovaginal fibronectin and cervical length at 23 weeks of gestation:relative risk of early preterm delivery. BJOG. 2000;107:1276-1281.

19. Guzman ER, Pisatowski DM, Vintzileos AM, et al. A comparison of ultrasonographically detected cervical changes in response to transfundal pressure, coughing, and standing in predicting cervical incompetence. Am J Obstet Gynecol. 1997;172:660-665.

20. Sonek J, Blumefeld M, Foley M, Johnson F, Iams J. Cervical length may change during ultrasonographic examination. Am J Obstet Gynecol. 1990;162:1355-1357.

21. Sonek JD, Iams JD, Blumefeld M, Johnson F, Landon M, Gabbe S. Measurement of cervical length in pregnancy: comparison between vaginal ultrasonography and digital examination. Obstet Gynecol. 1990;76:172-175.

22. Cook CM, Ellwood DA. A longitudinal study of the cervix in pregnancy using transvaginal ultrasound. BJOG. 1996;103:16-18.

23. Anderson HF. Transvaginal and transabdominal ultrasonography of the uterine cervix during pregnancy. J Clin Ultrasound. 1991;19:77-83.

24. Althuisius SM, Dekker GA, Hummel P, Bekedam D, van Geijn HP. Final results of the Cervical Incompetence Prevention Randomized Cervical Trial (CIPRACT): therapeutic cerclage with bed rest versus bed rest alone. Am J Obstet Gynecol. 2001;185:1106-1112.

25. Colombo DF, Iams JD. Cervical length and preterm labor. Clin Obstet Gynecol. 2000;43:735-745.

26. Heath VC, Souka AP, Erasmus I, Gibb DM, Nicolaides KH. Cervical length at 23 weeks’ gestation: the value of Shirodkar suture for the short cervix. Ultrasound Obstet Gynecol. 1998;12:318-322.

27. Hibbard JU, Snow J, Moawad AH. Short cervical length by ultrasound and cerclage. J Perinatol. 2000;20:161-165.

28. Hassan SS, Romero R, Maymon E, et al. Does cervical cerclage prevent preterm delivery in patients with short cervix? Am J Obstet Gynecol. 2001;184:1325-1331.

KEY POINTS

An extensive history is essential to identify the gravida at risk for a shortened cervical length.
Transvaginal measurement of cervical lengths less than 26 mm has a high predictive value for preterm delivery.
Prophylactic cerclage should be offered to patients with a classic history of cervical incompetence.
The benefits of therapeutic cerclage for a shortened cervix remain uncertain, especially in women deemed to have a low risk of preterm delivery.

You are in your busy office, running behind schedule, when you receive a frantic notice from the ultrasound department that your patient—a primigravida scheduled for a routine anatomy survey at 21 weeks’ gestation—has a cervical length of 19 mm with funneling. What are your management options if the patient reports no contractions or changes in vaginal discharge?

With today’s emphasis on evidence-based medicine, it often is difficult to decide on an appropriate course of action, especially when conflicting reports abound. This article reviews the best studies available and presents a practical algorithm (FIGURE 1) to guide management of this difficult dilemma.

FIGURE 1 Management strategy for women with risk factors for cervical incompetence, preterm delivery

Predicting the need for cerclage

Despite numerous explorations of its pathophysiology, no irrefutable data explain what triggers the cascade of events leading to preterm birth, which remains a leading cause of morbidity and mortality. Cervical incompetence has been studied extensively as a possible cause, often without conclusive results.

Traditionally, cervical incompetence is defined as dilatation in the absence of contractions, usually presenting in the second trimester.¹ Possible causes include prior obstetric or gynecologic injury, increased uterine volume (ie, multiple gestation, polyhydramnios), biological variation, and—currently under investigation—response to inflammatory mediators.²

In gravidas deemed to have biological or structural cervical incompetence, prophylactic cerclage is often placed at 12 to 14 weeks’ gestation. Alternatively, a therapeutic or “rescue” cerclage sometimes is placed at 20 weeks or more when evidence of cervical shortening or funneling is noted incidentally on ultrasound examination. At times, therapeutic cerclage becomes an emergency if prolapsing membranes are found.

With improvements in ultrasound imaging, transvaginal measurement of cervical length has become useful for guiding decisions on the need for cerclage.^3,4 Unfortunately, current studies report conflicting data on the outcomes of pregnancies managed with cerclage placement based on ultrasound imaging.^5-7

Ultrasound evaluation: Cervical length and other variables

In unselected general obstetric populations, preterm birth is associated with decreasing cervical length not only at 24 and 28 weeks’ gestation, but as early as 16 to 22 weeks’ gestation.^8-14 Decreasing cervical length also predicts preterm birth in women at risk for early delivery.^15,16 As a result, sonographic visualization of cervical length has evolved into a common practice to help predict early delivery.

An extensive history may offer clues as to what may have caused a shortened cervix in a previous pregnancy.

Iams et al¹⁷ compared cervical length with fetal fibronectin and noted that cervical length is more sensitive (39% versus 23%) but has a lower positive predictive value (PPV) (14% versus 20%). In combination, the modalities improved PPV to 50%. However, a separate study by Heath et al¹⁸ suggested that women with a cervical length of 15 mm or less had a relative risk of preterm birth 6 times greater than those with positive fetal fibronectin alone.

As sonographic technology has advanced, cervical characteristics have been assessed in a variety of ways. Besides measuring cervical length, experts also look for funneling, dilatation, prolapsing membranes, and dynamic changes elicited with transfundal or suprapubic pressure.^4,19,20

Although cervical length can be measured transabdominally and transperineally, transvaginal assessment is most accurate. Transvaginal measurements also are more predictive and reproducible than traditional digital cervical examinations.^16,21,22

Basic parameters. Standard cervical measurements use the “white stripe” of the internal cervical os as an anatomic landmark for proper caliper placement (FIGURE 2). As for dimensions of the nonlaboring cervix, Anderson²³ found an average length of 45±7 mm at 14 to 30 weeks, and Iams et al¹⁰ found a mean cervical length of 35±8 mm at 24 weeks’ gestation after studying more than 2,900 gravidas. In another investigation, Hibbard and colleagues found a mean cervical length of 38.5±8 mm at 20 weeks’ gestation.¹²

Cervical length and diameter should remain constant as gestation progresses in both primiparous and multiparous women.²²

Defining the short cervix. In a multicenter clinical trial, Iams et al¹⁰ measured the length of the cervix at 24 and 28 weeks in an unselected gravid population and determined that a length less than 26 mm at 24 weeks increases the relative risk for preterm delivery by a factor of 6.19, with a sensitivity of 37% for delivery before 35 weeks and a PPV of 18%—thus establishing the definition of a short cervix.