Guideline No. 401: Sonographic Cervical Length in Singleton Pregnancies: Techniques and Clinical Applications

Objectives: To assess the association between sonography-derived cervical length measurement and preterm birth. To describe the various techniques to measure cervical length using sonography. To review the natural history of the short cervix. To review the clinical uses, predictive ability, and utility of sonography-measured short cervix. Outcomes: Reduction in rates of prematurity and/or better identification of those at risk, as well as possible prevention of unnecessary interventions. Intended Users: Clinicians involved in the obstetrical management or cervical imaging of patients at increased risk of a short cervix. Target Population: Women at increased risk of a short cervix or at risk of preterm birth. Evidence: Literature published up to June 2019 was retrieved through searches of PubMed and the Cochrane Library using appropriate controlled vocabulary and key words (preterm labour, ultrasound, cervix, cervical insufficiency, transvaginal, transperineal, cervical length, fibronectin). Results were restricted to general and systematic reviews, randomized controlled trials, controlled clinical trials, and observational studies. There were no date or language restrictions. Grey (unpublished) literature was identified through searching the websites of health technology assessment agencies, clinical practice guideline collections, clinical trial registries, and national and international medical specialty societies. Values: The evidence and this guideline were reviewed by the Diagnostic Imaging Committee of the Society of Obstetricians and Gynaecologists of Canada, and the recommendations were made and graded according to the rankings of the Canadian Task Force on Preventive Health Care (Online Appendix Table A1). Benefits, Harms, Costs: Preterm birth is a leading cause of perinatal morbidity and mortality. Use of the sonographic technique reviewed in this guideline may help identify women at risk of preterm birth and, in some circumstances, lead to interventions that may reduce the rate of preterm birth. SUMMARY STATEMENTS (Canadian Task Force on Preventive Health Care grading in parentheses) 1. In the general obstetrical population, cervical length is relatively stable over the first 2 trimesters. The natural history of cervical length change may not be useful in identifying women at increased risk of spontaneous preterm birth. Because there may be different patterns, rates, and/or onset of cervical length shortening, repeat assessment of cervical length may be useful in patients at high risk of spontaneous preterm birth (II-2). 2. Transvaginal sonography can be used to assess the risk of preterm birth in women with a history of spontaneous preterm birth and to differentiate those at higher and lower risk of preterm delivery (II-2). 3. Cervical length measurement can be used to identify those at increased risk of preterm birth in asymptomatic women at <24 weeks gestation who have other risk factors for preterm birth (uterine anomaly or prior multiple dilatation and evacuation procedures beyond 13 weeks gestation). However, there is insufficient evidence to recommend specific management strategies in this group of women (II-2). 4. There is no consensus on the optimal timing or frequency of serial evaluations of cervical length. If repeat measurements are performed, they should be done at suitable intervals to minimize the likelihood of observation error (II-2). 5. No specific randomized trials have evaluated any interventions in asymptomatic women initially diagnosed at or beyond 24 weeks gestation who are at increased risk of preterm birth (e.g., those who have a history of prior spontaneous preterm birth, uterine anomaly, or prior multiple dilatation and evacuation procedures beyond 13 weeks gestation) and who have a short cervical length. However, knowledge of cervicanalal length beyond 24 weeks may help with empiric management strategies for these women, such as relocation and increased surveillance (III). 6. In women presenting with suspected preterm labour and intact membranes, transvaginal sonographic assessment of cervical length may be used to help stratify the risk of preterm delivery and prevent unnecessary intervention without harm. This information may result in a reduction in late preterm birth, but it is unclear whether it makes a significant clinical difference (II-2B). 7. Cervical length surveillance is a safe option for patients with a prior sonography indicated cerclage, unclear history of cervical insufficiency, and prior spontaneous preterm birth when compared with routine cerclage based on clinical assessment; it may reduce the need for subsequent cerclage (II-2B). 8. Transvaginal sonography appears to be safe in preterm prelabour rupture of membranes, but its clinical predictive value is uncertain in this context (II-2). 9. There is insufficient evidence to support a committee position on the frequency or timing of sonographic cervical length assessment post cerclage. It is unclear if there is significant clinical benefit of such scans (III). 10. Sonographic cervical length assessment and fetal fibronectin appear to be similar in predictive ability in symptomatic patients, and their combined value may not be significantly different from assessment of cervical length alone. More research is needed in this area (II-2).