An insertion detector for monitoring a position of a medical probe relative to a body cavity of a patient, the probe incorporates a proximity sensor that is responsive to a predetermined property of the patient's body. The proximity sensor may include a light emitter and a light detector. When the medical probe is inserted into the body cavity, a light flux between the light emitter and light detector is changed due to either obstruction by the cavity walls or reflection by the patient's skin. A response from the proximity sensor may be used to adjust a temperature measured from the body cavity to correct for errors due to non-insertion or partial insertion of the probe into the body cavity.