1. An ultrasonic diagnostic imaging system for analyzing the fetal heart comprising: a 3D imaging probe having a two-dimensional matrix array transducer operable to acquire image data sets of the fetal heart; a user control which is manipulated to designate a location of motion in the anatomy of an ultrasound image which is representative of the fetal heart cycle; a motion estimator, responsive to echo signals from the designated location, which processes the echo signals by Doppler processing, speckle tracking or block matching to produce an estimate of the motion of moving fetal tissue or blood for production of a gating signal in relation to the fetal heart cycle; a probe controller, responsive to the gating signal, which causes the two-dimensional matrix array transducer of the 3D imaging probe to acquire 3D data sets at one or more predetermined phases of the fetal heart cycle as determined by the gating signal; and a display responsive to the 3D data sets for producing a 3D image of the fetal heart.