The National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network has completed 2 prospective, blinded observational studies of cervical ultrasound for the prediction of preterm birth and the identification of high-risk women who might benefit from interventions aimed at preventing spontaneous preterm birth. These reports and other clinical data support the concept that cervical competence is not a dichotomous variable, but more likely functions along a continuum that is reflected by the relationship between cervical length and reproductive outcomes. Because of its safety, availability, patient acceptance and reproducibility, the sonographic evaluation of the uterine cervix has become an important investigational tool that has advanced our understanding of the preterm birth syndrome and should allow a more focused approach to the most persistent and challenging problem in modern obstetrics.