A body compatible fiber optic sensor probe for invasive medical use is provided. The probe includes at least one sensing location at which the fiber is configured to provide at least one detectable changeable optical property responsive to strain within the fiber, and at least one sensing element which undergoes a volumetric change in response to an in body parameter to be sensed. The sensing element is coupled to the fiber in such a way that the volumetric change induces strain within the fiber so as to vary the detectable optical property or properties.