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The Role of Routine Cervical Length Screening For Preterm Birth Prevention


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
    • 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”
    • 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 


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.


  • 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