Abstract: Systems, methods, and apparatuses for non-invasively measuring volumes of a body cavity by a wearable ultrasonic device are provided. The wearable ultrasonic device can collect data related to the body cavity perform diagnostic measurements associated with the body cavity to provide alerts to user. The wearable ultrasonic device can be affixed to the skin of a user's body. The wearable ultrasonic device can include an ultrasonic transducer that transmits and receives ultrasonic signals directed toward the body cavity such as a user's urinary bladder. The device can include processing electronics coupled to the ultrasonic transducer to process the ultrasonic signals to determine the volume of the body cavity and transmit alerts to the user when the volume reaches a particular threshold. The processing electronics can be integrated with the ultrasonic transducer to operate at low voltages and be battery powered.