The Role of Routine Cervical Length Screening For Preterm Birth Prevention

CLINICAL ACTIONS:

Professional organizations have released guidance on cervical length (CL) screening for preterm birth (PTB) prevention. The finding of a short cervix, irrespective of obstetric history, has been consistently shown to be associated with higher risk for PTB.

Steps for CL assessment

• Have patient empty bladder
• Acquire the sagittal, long-axis image of entire cervix (75% of screen)
• Confirm that the anterior and posterior cervixes are of equal thickness
• Measure endocervical canal from internal to external os
  • Ensure that internal and external os are seen and canal seen throughout
  • Place calipers at the internal and external os where the anterior and posterior walls of the cervix meet
• If the endocervical canal curves
  • Add linear measurements together to obtain the final cervical length
• Dynamic cervical shortening
  • Exam time 3–5 minutes and/or
  • suprapubic/fundal pressure
• Obtain 3 images for CL determination
  • Use the shortest, best measurement

Who and When to Screen for CL

Screening in Asymptomatic women

• ACOG
  • Uncertainty regarding clinical utility of endovaginal cervical length screening
    • Incidental finding of short cervix is associated with increased risk and vaginal progesterone may be of benefit 
  • Cervix should be visualized as part of 18w0d to 22w6d anatomy scan using either abdominal or endovaginal approach 
  • If a short cervix is found on abdominal scan, endovaginal ultrasonography is recommended
• SOGC (Canada)
  • Universal screening in Canada can not be mandated at this time and more research is warranted
• ISUOG
  • CL measurements should be done by transvaginal scanning that would require additional consent, appropriate training and result auditing 
  • Based on the above, “CL measurement can be carried out meeting these conditions, it can be considered as an integral part of the routine mid-trimester scan”
• RANZCOG
  • Cervical length measurement at 18-24 weeks should be “considered”
  • Two step approach of abdominal scan first followed by transvaginal as needed
• FIGO
  • Does recommend measurement in low risk population
  • “Should be performed in all pregnant women at 19–23+6 weeks of gestation using transvaginal ultrasound”
  • Can be done same time as anatomic scan

Surveillance in asymptomatic women with prior sPTB

• ACOG: Every 1 to 4 weeks (depending on clinical status) between 16w0d and 24w0d 

SYNOPSIS:

Two thirds of preterm births are spontaneous, and only 10% of births <34 weeks will occur to women with a history of spontaneous preterm birth (sPTB). Few predictive tests are available for PTB prediction, and mid-trimester CL assessment remains the best clinical tool at identifying high-risk women. Women with short cervix and prior sPTB are at highest risk.

KEY POINTS:

• If cervical length is 25 mm or less
  • “Options for intervention include cerclage or vaginal progesterone”
• Transvaginal ultrasound is more sensitive and reproducible that transabdominal ultrasound in detecting cervical shortening
• Sonographers and practitioners should receive specific training in acquisition and interpretation of cervical imaging

Special circumstances

• History of treatment for cervical dysplasia (LEEP, cold-knife cone)
  • Observational data inconsistent
  • Risk may be increased if excision greater than 15 mm in depth and short time between surgery and pregnancy 
• Multiple gestations
  
  • ACOG does not recommend CL surveillance due to lack of effective interventions
  • As with average risk population, if short cervix found on transabdominal ultrasound, proceed to endovaginal ultrasound 

Learn More – Primary Sources:

ACOG Practice Bulletin 234: Prediction and Prevention of Spontaneous Preterm Birth

ISUOG Practice Guidelines (Updated): Performance of the routine mid-trimester fetal ultrasound scan

Fetal Medicine Foundation Cervical Length Assessment Program

SOGC: Universal Cervical Length Screening

FIGO: Best Practices in Maternal Fetal Medicine 

ISUOG: How to Measure Cervical Length