The causality between late abortion respectively premature birth and cervical insufficiency is proven. The prophylactic and therapeutic cerclage can be a contribution to a reduction of the frequency of premature births. A sound knowledge of the physiological state of the cervix uteri during pregnancy is necessary for a well-timed indication. This problem was confirmed by a study with 258 longitudinal examinations (134 nulliparae, 124 parae). There was no significant difference between the two groups of patients. The change of the cervix uteri is described in 45 cases of cervical insufficiency without labour. In most of the cases the disease developed from a physiological initial or intermediate stage within a short time. Short-term controls are necessary in case of divergency from the physiological state.