The Role of Routine Cervical Length Screening For Preterm Birth Prevention

CLINICAL ACTIONS:

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
  • ISUOG
    • Insufficient evidence to recommend offering universal screening in the general population
  • 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

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

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:

  • 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

AIUM Practice Parameter for the Performance of Limited Obstetric Ultrasound Examinations by Advanced Clinical Providers

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