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
    • 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

  • 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

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

CLINICAL ACTIONS:

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

SYNOPSIS:

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.

KEY POINTS:

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