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