1. An ultrasound diagnostic apparatus comprises: an ultrasound probe configured to transmit ultrasound waves to a subject and receive echo signals; an ultrasound image display control device configured to display a real-time ultrasound image based on the received echo signals, wherein the real-time ultrasound image includes an ultrasound cross-section that includes a characteristic region of the subject; a location sensor configured to detect a location of the ultrasound probe; a location calculation device configured to calculate a first location of the echo signal in a first coordinate system of the real-time ultrasound image based on the location of the ultrasound probe; an input device configured to receive an input to display a reference cross-section of a reference medical image, wherein the input includes a selection of the reference cross-section of the reference medical image, by an operator, from among a plurality of reference cross-sections of reference medical images previously acquired, wherein the selected reference cross-section of the reference medical image identifies the characteristic region of the subject in the reference cross-section of the reference medical image and identifies a second location of the reference cross-section in relation to the subject, and wherein the reference medical image is a medical image other than the real-time ultrasound image; and a reference medical image display control device configured to display the reference cross-section of the reference medical image based on the plurality of reference cross-sections of reference medical images previously acquired, wherein the reference cross-section is displayed according to the input.