Abstract: An apparatus for measuring cervical diameter comprises a support structure and measurement devices for detecting changes in cervical diameter, either directly or indirectly through changes in the size of the support structure. The support structure may conform to a cervical surface, typically being a peripherally expansible lumen or expansible structure. Alternatively, the support structure may engage the vaginal wall or fornices. Measurement devices may include gages which determine change in sizes of an expansible loop, electronic devices for measuring changes in transmitted or reflected energy, or combinations thereof. The devices are suitable for use on ambulatory patients and in out-patient situations.