Continuous cervical dilatation monitoring by ultrasonic methods during labor.

Continuous recording of cervical dilatation during labor has been investigated in 13 pregnancies. The recordings were obtained with an ultrasonic cervimeter that continuously monitors cervical dilatation from the transit time of ultrasound signals between two piezoelectric crystals attached on the uterine cervix. A small spring-loaded clip allowed each crystal to be fixed on the rim of the cervical os. Clinical accuracy was +/- 0.6 cm. When the ultrasound recording of cervical dilatation is compared to the intrauterine pressure curve, it is characterized by a baseline and wave-shape curve of dilatation (DWP). The maximal amplitude component is called cervical maximal plasticity. The onset of the DWP is related to cervical resistivity, and the end of DWP reflects the relaxation time of cervical dilatation. The data show that as dilatation enters the active phase of labor, the plasticity, the resistivity, and the duration of relaxation of the cervix increase. These observations are discussed and related to the structural changes of the cervix during labor.