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The Role of Routine Cervical Length Screening For Preterm Birth Prevention


SMFM and other 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

Note: SMFM does not recommend routine CL surveillance in women with cerclage, multiple gestation, PPROM, or placenta previa (Grade 2B)

Who and When to Screen for CL

Screening in Asymptomatic women

  • SMFM
    • Based on evidence, general population screening “cannot yet be universally mandated”
    • For those who wish to screen their patients including low risk, it may be considered ‘reasonable’ with
      • “Strict adherence to guidelines”
      • Measurement between 16 and 24 weeks (Grade 2B)
  • 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

  • SMFM: Every 1 to 2 weeks (depending on clinical status) between 16 and 24 weeks (Grade 1A)


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.


  • Current threshold for “short” cervix: Range of 20 to 30 mm
  • 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 (Grade 2B)
  • Practitioners who implement universal screening should follow strict guidelines (Grade 2B)

Special circumstances

  • History of treatment for cervical dysplasia (LEEP, cold-knife cone)
    • CL screening the same as that of asymptomatic women without history of sPTB
  • Determining status post-cerclage placement
    • Insufficient data to suggest clinical benefit (Grade 2B)
  • Multiple gestations
    • 18% will have short cervix by 22-24 weeks
    • SMFM does not recommend CL surveillance due to lack of effective interventions (Grade 2B)
  • Threatened preterm labor (PTL)
    • CL measurement may be used as adjunct to digital examination
    • In conjunction with FFN in women with CL of 20-29 mm (‘grey zone’)
      • FFN negative: no treatment
      • FFN positive: corticosteroids and additional interventions
    • PPROM
      • Insufficient data to suggest clinical benefit (Grade 2B)
    • Placenta previa
      • Insufficient data to suggest clinical benefit (Grade 2B)

Learn More – Primary Sources:

SMFM Consult Series #40: The role of routine cervical length screening in selected high-and low-risk women for preterm birth prevention

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