1. An ultrasonic diagnostic imaging system for analyzing a fetal heart, the system comprising: a single 3D ultrasound imaging probe having a two-dimensional matrix array transducer and a micro-beamformer operable to acquire 3D image data sets of the fetal heart and M mode echo signals from a selectably positioned M line intersecting the fetal heart, the 3D imaging probe configured to transmit beams along the M line and acquire the M mode echo signals generated in response to the transmitted beams; a user control configured to, in response to user input, selectably position the M line by positioning an M line cursor which designates a location of motion in fetal heart anatomy in a displayed ultrasound image which is representative of a fetal heart cycle; a motion estimator responsive to the acquired M mode echo signals from the designated location in the fetal heart anatomy, the motion estimator configured to process the M mode echo signals to produce an estimate of motion of moving fetal tissue or blood and produce a gating signal in relation to the fetal heart cycle based on the estimated motion of the moving fetal tissue or blood; a probe controller responsive to the gating signal and configured to cause the two-dimensional matrix array transducer of the 3D imaging probe to acquire the 3D image data sets at one or more predetermined phases of the fetal heart cycle as determined by the gating signal; an image processor configured to receive the 3D image data sets and process the received 3D image data sets to generate a 3D image of the fetal heart; and a display configured to display the 3D image of the fetal heart.