An ultrasound diagnostic apparatus (1, 2) comprising: a three-dimensional data generation unit configured to generate three-dimensional data for one or more regions in a body of a subject based on reflected waves reflecting back from the body of the subject after ultrasound waves have been transmitted towards the body of the subject; a measurement image selection unit configured to select, based on an intensity of the reflected waves, one of two-dimensional cross-sections that compose the three-dimensional data, as a measurement reference image used for measuring a length of each region in the body of the subject; a measurement and calculation unit configured to measure the length of each region in the body of the subject using the selected measurement reference image, and to calculate an estimated weight of the subject using the measured lengths; and an output unit configured to output the calculated estimated weight, characterised in that said measurement image selection unit includes: a hyperechoic region extraction unit configured to extract, from the three-dimensional data, a hyperechoic region which is a region corresponding to the reflected waves having a reflection intensity that is greater than a threshold value; a cut plane obtainment unit configured to obtain two-dimensional cross-sections that compose the three-dimensional data, by cutting the three-dimensional data based on a three-dimensional feature of the extracted hyperechoic region; and a reference image selection unit configured.to select one of the two-dimensional cross-sections as the measurement reference image used for measuring the length of the region in the body of the subject. a three-dimensional data generation unit configured to generate three-dimensional data for one or more regions in a body of a subject based on reflected waves reflecting back from the body of the subject after ultrasound waves have been transmitted towards the body of the subject; a measurement image selection unit configured to select, based on an intensity of the reflected waves, one of two-dimensional cross-sections that compose the three-dimensional data, as a measurement reference image used for measuring a length of each region in the body of the subject; a measurement and calculation unit configured to measure the length of each region in the body of the subject using the selected measurement reference image, and to calculate an estimated weight of the subject using the measured lengths; and an output unit configured to output the calculated estimated weight, a hyperechoic region extraction unit configured to extract, from the three-dimensional data, a hyperechoic region which is a region corresponding to the reflected waves having a reflection intensity that is greater than a threshold value; a cut plane obtainment unit configured to obtain two-dimensional cross-sections that compose the three-dimensional data, by cutting the three-dimensional data based on a three-dimensional feature of the extracted hyperechoic region; and a reference image selection unit configured.to select one of the two-dimensional cross-sections as the measurement reference image used for measuring the length of the region in the body of the subject.