1. An ultrasonic diagnostic apparatus with a scanning device having a plurality of ultrasonic transducers arranged in a predetermined direction, for acquiring a three-dimensional ultrasonic image by scanning an ultrasonic wave in a three-dimensional imaging region of an object to be examined while the ultrasonic transducers are oscillated in a direction orthogonal to a scanning plane corresponding to the predetermined direction, comprising: a two-dimensional ultrasonic image acquiring device configured to acquire a two-dimensional ultrasonic image of a head of an unborn baby; an unborn baby development data obtaining device configured to obtain data relating to development information of an unborn baby based on a diameter of the acquired two-dimensional ultrasonic image of the head of the unborn baby having a longest lateral width; an ROI (region of interest) determination device configured to determine a size of a region of interest on the scanning plane, in proportion to a three dimensional size of the head of the unborn baby by determining the three dimensional size, including lateral width, longitudinal width, and width in a depth direction, of the head of the unborn baby based on the data relating to the development information of the unborn baby obtained by the unborn baby development data obtaining device; an oscillation device configured to oscillate the ultrasonic transducers through a variable angle range; and an angle range determination device configured to determine an angle range for oscillating the ultrasonic transducers in the direction orthogonal to the scanning plane, based on the determined size of the region of interest, wherein the scanning device is configured to acquire a three-dimensional ultrasonic image by scanning in an angle range determined according to the angle range determination device.