Changes in the mechanical forces of cervical distention before and after rupture of the membranes.

Rupture of the membranes at term, when performed at an appropriate time, is clinically known to induce or accelerate labor. In the present study, the mechanisms of cervical softening and onset and/or enhancement of labor after spontaneous or artificial rupture of the membranes were investigated. The mechanical forces involved in cervical distention at the area where the fetal head is in contact with the cervix were mathematically estimated before and after rupture of the membranes, and the difference between forces under both conditions was calculated. Rupture of the membranes, when occurring either spontaneously or artificially at an appropriate time, has been found to result in an increase of the force of distention in the cervix where it comes into contact with the head. Thus, the increased force against the cervix, as seen by stretching, through the possible increased release of prostaglandins, results in cervical softening and initiation and/or enhancement of labor.