Sonographic techniques have been developed to study the biometry and behavior of the cervix during pregnancy which have led to a better understanding of the pathogenesis of pregnancy loss and preterm birth. Nomograms have been developed for singleton and twin gestations, and associations between cervical sonographic characteristics and various adverse pregnancy outcomes have been observed. The new concept of 'relative' cervical incompetence suggests that the cervix may be an independent contributor to preterm birth. The incorporation of cervical sonography in management schemes for the prevention of pregnancy loss or preterm delivery has the potential to impact favorably on perinatal outcome.