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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

Cervical Cerclage – Professional Recommendations


ACOG defines cervical insufficiency as “the inability of the uterine cervix to retain a pregnancy in the absence of the signs and symptoms of clinical contractions, or labor, or both in the second trimester.” In addition, ACOG separates out indication for cerclage in to 3 categories

  • History: ≥1 of the following
    • Second trimester pregnancy losses related to painless cervical dilation and no history of labor or abruption
    • Previous second trimester cerclage for painless cervical dilation
  • Physical Examination: Also known as ‘physical examination–indicated cerclage’, ‘rescue cerclage’ and ’emergency cerclage’
    • Patient presents with painless second trimester cervical dilation
  • Ultrasound: Cervical length shortening and history of preterm birth
    • Singleton pregnancy
    • Prior spontaneous preterm birth (<34 weeks)
    • Cervical length:  <25 mm (at <24 weeks)

SMFM states that cerclages are effective in woman based on the following indications:

  • History indicated: 3 or more PTBs or second-trimester losses.
  • Ultrasound indicated: ≥1 early PTB (defined as delivery between 17w to 33w6d), and cervical length (CL) <25 mm on transvaginal ultrasound (TVUS) before 24 weeks

Timing of Cerclage Placement

  • History-indicated cerclage
    • Place between 12 and 14 weeks after confirmation of pregnancy viability
  • Ultrasound or exam-indicated cerclage
    • May be placed prior to 23 weeks

Risk Factors

  • Prior PTB
  • Repeated cervical dilation
  • Cervical procedures (including cone and LEEP)
  • Cervical laceration
  • Urogenital abnormalities


Clinically, cervical insufficiency is painless dilation and recurrent mid-trimester losses without signs of preterm labor (PTL), PPROM, or infection. Patient history may include superimposed symptoms (i.e. bleeding, pressure), therefore a judicious review of records is advised.  Those with a history of prior preterm birth can benefit from cervical length screening to appropriate guide selected patients for cerclage.


Ultrasound Indicated Cerclage with Prior History of Preterm Birth or Second Trimester Losses (SMFM)

  • CL surveillance
    • Begin at 16 weeks
    • Perform every 2 weeks
    • CL measurement 25-29 mm: Perform weekly
  • Offer cerclage when
    • CL <25 mm prior to 23w0d weeks and
    • History of spontaneous PTB at 17w0d to 33w6d
  • 17–alpha hydroxyprogesterone caproate (17-OHPC)
    • Note: Cerclage placement or presence should not alter recommendations
  • Evidence from research studies
    • There is no difference in efficacy of McDonald versus Shirodkar techniques

Special Circumstances

  • Diagnosis of cervical insufficiency is unclear
    • Consider close screening starting at 16 weeks in place of history indicated cerclage placement
  • Very early losses
    • Consider beginning screening <16 weeks if there is a history of very early second trimester losses

‘Emergency’ Cerclage (Exam indicated)

  • There is literature, including a meta-analysis (Obstet Gynecol, 2015), to support ’emergency’ or ‘rescue’ cerclage
    • Neonatal survival
      • Cerclage: 71% survival
      • No cerclage: 43% survival
      • Relative risk 1.65 (95% CI 1.19–2.28)
    • Prolongation of pregnancy
      • Mean difference: 33.98 days (95% CI, 17.88 to 50.08)
    • Authors note significant limitations including quality of data and only 1 RCT included

After clinical examination to rule out uterine activity, or intraamniotic infection, or both, physical examination-indicated cerclage placement (if technically feasible) in patients with singleton gestations who have cervical change of the internal os may be beneficial

SMFM Choosing Wisely Campaign

  • SMFM recommends the following after cerclage placement
    • Do not perform serial cervical length measurements
    • There is no evidence that cervical length monitoring following a cerclage improves outcomes, despite cervical shortening being associated with increased risk for preterm birth

Learn More – Primary Sources:

SMFM: Cervical cerclage for the woman with prior adverse pregnancy outcome

ACOG Practice Bulletin 142: Cerclage for the Management of Cervical Insufficiency

Physical Examination–Indicated Cerclage: A Systematic Review and Meta-analysis