A system for characterizing a patient uterus, comprising: a source (140A, 405) of a pressurized flow of a fluid; a probe for transcervical introduction into a patient's uterine cavity; a lumen mounted to the probe; a sensor; a valve and a pressure regulator for allowing a flow of gas from the source at a predetermined flow rate and pressure to the lumen; the probe having an elongate introducer member for accessing a patient's uterine cavity, the elongate introducer member having a working end; the working end including a flexible, expandable thin-wall member or structure of a dielectric material with a fluid-tight interior chamber that when expanded has a shape extending into substantially all regions of the patient's uterus, and configured for contacting the patient's endometrial lining that is targeted for ablation; and a controller (140B;410); characterized in that: the introducer member comprises a balloon for occluding the cervical canal of the patient.; the sensor is a flow sensor for monitoring a change in flow rate of the fluid from the lumen into the uterine cavity; the lumen is configured to deliver the fluid from the source to the exterior surfsaces of the expanded dielectric structure to fill the uterine cavity; the controller being operatively coupled t the flow sensor and configured to characterize the uterine cavity as at least one of perforated or non-perforated based on the change in the rate.. a source (140A, 405) of a pressurized flow of a fluid; a probe for transcervical introduction into a patient's uterine cavity; a lumen mounted to the probe; a sensor; a valve and a pressure regulator for allowing a flow of gas from the source at a predetermined flow rate and pressure to the lumen; the probe having an elongate introducer member for accessing a patient's uterine cavity, the elongate introducer member having a working end; the working end including a flexible, expandable thin-wall member or structure of a dielectric material with a fluid-tight interior chamber that when expanded has a shape extending into substantially all regions of the patient's uterus, and configured for contacting the patient's endometrial lining that is targeted for ablation; and a controller (140B;410); characterized in that: the introducer member comprises a balloon for occluding the cervical canal of the patient.; the sensor is a flow sensor for monitoring a change in flow rate of the fluid from the lumen into the uterine cavity; the lumen is configured to deliver the fluid from the source to the exterior surfsaces of the expanded dielectric structure to fill the uterine cavity; the controller being operatively coupled t the flow sensor and configured to characterize the uterine cavity as at least one of perforated or non-perforated based on the change in the rate.. the introducer member comprises a balloon for occluding the cervical canal of the patient.; the sensor is a flow sensor for monitoring a change in flow rate of the fluid from the lumen into the uterine cavity; the lumen is configured to deliver the fluid from the source to the exterior surfsaces of the expanded dielectric structure to fill the uterine cavity; the controller being operatively coupled t the flow sensor and configured to characterize the uterine cavity as at least one of perforated or non-perforated based on the change in the rate..