ULTRASOUND IN INCOMPETENT CERVIX  

 

Normal Cervical Length (1). Transvaginal scanning.

  • 24 weeks
    • nulliparous women = 34.0 ± 7.8 mm.
    • parous women = 36.1 ± 8.4 mm.
  • 28 weeks
    • nulliparous women = 32.6 ± 8.1 mm.
    • parous women = 34.5 ± 8.7 mm.

 

 

 

 

 

 

 

 

A closed cervix is a normal finding during pregnancy. The endocervical canal appears as either a hyperechoic, or less commonly a hypoechoic band within the cervix. The lower uterine segment and cervix have a Y-shaped configuration. The internal os may vary in appearance from a flat to a slightly funnel shaped appearance. The membranes are closely applied to the internal os (3).


 

ULTRASOUND CRITERIA
  • Ultrasound Criteria for the diagnosis of incompetent cervix.
  • The shorter the cervix at 24 or 28 weeks the greater the risk of preterm birth.
  • Patients with a cervical length below 22mm (5th percentile) had a 20% risk of preterm delivery (1). This has a poor positive predictive value (i.e. intervention would involve 4 normal patients for every patient that is really at risk).
  • Funneling of the internal os at any gestation is associated with a high risk of preterm delivery.
  • "Hourglass membranes" represents a dilated endocervical canal caused by prolapse of the amniotic sac. Spontaneous abortion usually occurs and occasionally fetal parts may be seen in the cervix (2).

 

 

Video clip of Cervical Imcompetence- Funneling

Video clip of incompetent Cervix

 

 

 

  • There is a significant correlation between the Bishop score (digital examination of the cervix) and the length measured at ultrasound (1).
    • Negative predictive value = 96%.
    • A firm, uneffaced, non dilated cervix on digital examination had a risk of <4 % of preterm birth i.e. ultrasound will not add any useful additional information on the risk of preterm delivery when the cervix is long and closed on digital examination.
    • The converse is probably not true i.e. when digital examination reveals a short and effaced cervix, the cervical length on transvaginal ultrasound may be ³3 cm.
  • Heath and co-workers (4) studied women who are at increased risk of preterm birth and using transvaginal ultrasound at 23 weeks found that:
    • 1.7% had a cervical length less than or equal to 15mm,
    • These women accounted for 90% of deliveries at less than 28 weeks and 60% of deliveries at 32 weeks or less.
    • This suggests that the positive predictive value for a short cervix (15mm or less) is much greater for extreme prematurity (28 weeks or less).
    • The authors have created a formula to predict the risk of spontaneous delivery at 32 weeks or less based on cervical length at 23 weeks.

·         In singleton pregnancies, the rate of spontaneous delivery before 33 weeks is 1-2% and the risk of such an early delivery can be predicted from the measurement of cervical length at 23 weeks of gestation (5,6).  The estimated risk increases exponentially with decreasing cervical length from about:

o         0.2% at 60 mm

o         0.8% at 30 mm

o         4.0% at 15 mm

o         78% at 5 mm (6).

  • In twin pregnancies the rate of preterm delivery before 33 weeks is 5-10% (7) and recent evidence suggests that measurement of cervical length at 23 weeks of gestation provides useful prediction of early preterm delivery (8-10).  In a study of 215 twin pregnancies examined at 23 weeks the estimated risk for early preterm delivery increases exponentially with decreasing cervical length from about:
    •  2% at 55 mm
    •  4% at 40 mm
    • 30% at 20 mm
    • 70% at 10 mm (10).
  • Transvaginal scanning is the method of choice. Scanning is done with the urinary bladder empty, and the tip of the probe is 2-3 cm from the external os. Scanning should be performed in both longitudinal and transverse planes.
  • Translabial may be used if the membranes have ruptured (minimizing the risk of infection).

 

PITFALLS IN THE DIAGNOSIS

 

Pitfalls in the Diagnosis of Incompetent Cervix

 

 

REFERENCES
  1. Iams JD, Goldberg RL, Meis PJ et.al. N Engl J Med 1996;334:567-572.
  2. McGahan JP. Phillips HE, Bowen MS. Prolapse of the amniotic sac ("hourglass membrane"). Radiology 1982;140:463-466.
  3. Sarti DA, Semple WF, Hobel CJ et.al. Ultrasonic visualization of a dilated cervix during pregnancy. Radiology 1979;130:417-420.
  4. Heath VC, Southhall TR, Souka AP et.al. Cervical length at 23 weeks of gestation: prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol 1998;12:312-317.
  5. Iams JD, Goldenberg RL, Meis PJ, Mercer BM, Moawad A, Das A et al. The length of the cervix and the risk of spontaneous delivery. N Engl J Med 1996; 334: 567 - 572
  6. 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
  7. Sebire NJ, Snijders RJM, Hughes K, Sepulveda W, Nicolaides KH. The hidden mortality of monochorionic twin pregnancies. Br J Obstet Gynaecol 1997; 104: 1203 - 1207
  8. Imseis HM, Albert TA, Iams JD. Identifying twin gestations at low risk for preterm birth with a transvaginal ultrasonographic cervical measurement at 24-26 weeks' gestation. Am J Obstet Gynecol 1997; 177: 1149 - 1155
  9. Goldenberg RL, Iams JD, Miodovnik M, Van Dorsten JP, Thurnau G, Bottoms S et al. The preterm prediction study: risk factors in twin gestations. Am J Obstet Gynecol 1996; 175: 1047 - 1053
  10. Souka AP, Heath VCF, Flint S, Sevastoploulou I, Nicolaides KH. Cervical length at 23 weeks in twins in predicting spontaneous preterm delivery. Obstet Gynecol 1999; 94: 450 -454