Monitoring the effacement of the uterine cervix by transperineal sonography: a new perspective.

The objective of this study is to monitor the process of effacement of the uterine cervix and demonstrate that transperineal sonography is the appropriate technique for this purpose. Eighty-six patients with normal, term pregnancies were studied at the beginning of labor. Transperineal sonography was performed in transverse and longitudinal planes. After the initial examination, patients were reexamined several times during a 1 to 4 hour period. We observed a progressive shortening of the canal and the synchronous opening of a funnel-shaped internal cervical os. When the funneling process reached the lower end of the cervix, both orifices fused, completing the process of effacement. The dilatation of the external os, which remained stationary during initial phase, increases very quickly once the effacement has been completed. Transperineal sonography efficiently imaged the changes described here.