Abstract: A flexible elastomeric annulus-shaped membrane having a shape-retentive memory and exerting a force so as to assume and to maintain a predetermined closed-loop geometric shape, normally a circle, fits circumferentially about the cervix os of a human female so as to hold and retain medical instrumentation probes, preferably two opposed wire-connected ultrasonic transducers of a real-time transit-time ultrasonic monitor of cervical dilatation and effacement. The annular membrane may optionally extend as a tube downwards in the vaginal canal, in the manner of a female diaphragm, as to shield the wires from the walls of the vagina. The membrane expands and contracts with such cyclical variation in the dilatation and effacement of the cervix os as occurs from the earliest onset of labor until imminent childbirth.