It is well recognized that preterm birth is the leading cause of perinatal mortality and morbidity. There is a significant association between cervix length and preterm birth risk. Most authorities consider a cervical length <3 cm as the lower limit of normal. A cervical length >3 cm has a high negative predictive value for delivery less than 34 weeks. A cervical length of <15 mm is moderately predictive (∼ 70%) of preterm birth within 48 hours. Cervical length is normally distributed and should remain relatively constant until the third trimester. Transabdominal US is the least reliable method of cervical length assessment. The most reliable method of documenting cervical length is transvaginal ultrasound (TVUS). Transperineal US is an alternative for imaging if TVUS is contraindicated, such as with premature rupture of membranes. However, the resolution is decreased compared to TVUS. Short cervix length is the single most important predictive finding for premature delivery. This observation should prompt consultation for high risk obstetrical care and consideration of other management options such as cerclage or activity restriction.The ACR Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed biennially by a multidisciplinary expert panel. The guideline development and review include an extensive analysis of current medical literature from peer reviewed journals and the application of a well-established consensus methodology (modified Delphi) to rate the appropriateness of imaging procedures by the panel. In those instances where evidence is lacking or not definitive, expert opinion may be used to recommend imaging.