The introduction of the ultrasonographical examinations into the obstetrical diagnostics has created further possibilities in the process of recognition of the isthmico-cervical insufficiency. The qualification of the opening forms of the internal cervical os, described as Y, V and U forms, is possible with the utilization of transperineal and, especially, transvaginal sonography. The aim of this study was the valuation of the ultrasonographical examinations' usefulness in the diagnostics of the isthmico-cervical insufficiency and the estimation of the opening form of the internal cervical os as a prognostic factor in the pregnancy course. 265 cases of pregnant women hospitalized in Department of Fertility and Obstetrics Medical University of Wrocław have been analyzed. On the ground of the executed examinations it has been ascertained that the ultrasonographical examination, considering the valuation of the opening form of the internal cervical os, is essential in the diagnostics of the isthmico-cervical insufficiency. Furthermore, it has been proved that in the aspect of the obtainment of the full-term pregnancy, the worst-prognosing form of the internal cervical os is the U form and the statement of a U or V formed opening confirms the necessity of the circular suture of the cervix.