1. An ultrasonic diagnostic apparatus comprising: a probe configured to transmit and receive ultrasound to and from a heart of a fetus; a display; a processor configured to form an ultrasonic image of the heart based on a reception signal obtained by transmission and reception of the ultrasound to and from the heart; said processor configured to set, on the ultrasonic image of the heart, proximal and distal observation points on conduction pathways of the heart for an electric signal which causes contraction of a cardiac muscle, wherein the proximal observation point is near a source of the electric signal in the heart, and the distal observation point is farther away from the source of the electric signal than the proximal observation point; said processor configured to track each of the proximal and distal observation points based on the reception signal or the ultrasonic image to determine temporal displacements of each of the proximal and distal observation points; said processor configured to generate a proximal displacement waveform representing the temporal displacements for the proximal observation point and a distal displacement waveform representing the temporal displacements for the proximal observation point; said processor configured to display, on said display, a displacement waveform image comprising the proximal displacement waveform and the distal displacement waveform arranged in a predetermined order in parallel to a common time axis; said processor configured to apply a sorting process to the proximal displacement waveform and the distal displacement waveform by heartbeat period; said processor configured to detect a representative point representing a proximal contraction timing in each of heartbeats on the proximal displacement waveform which has been sorted by the sorting process and a representative point representing a distal contraction timing in said each of the heartbeats on the distal displacement waveform which has been sorted by the sorting process, wherein the proximal contraction timing is a generation timing of local contraction caused by the electric signal at the proximal observation point, and the distal contraction timing is a generation timing of local contraction caused by the electric signal at the distal observation point; said processor configured to calculate a time difference of motion for said each of the heartbeats based on the plurality of representative points in said each of the heartbeats; the processor configured to display, on said display, the time difference of motion of said heartbeats; said processor configured to connect the plurality of representative points in said each of the heartbeats to generate a plurality of connecting curves; and said processor configured to display, on said display, the plurality of connecting curves across the proximal and distal displacement waveforms.