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. 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. Women at increased risk of a short cervix or at risk of preterm birth. 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 Grey (unpublished) literature was identified through searching the websites of health technology assessment agencies, clinical practice guideline collections, clinical trial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. 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). 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.