1. An ultrasonic diagnostic imaging system for gated acquisition of 3D fetal heart images comprising: an ultrasound probe including a two dimensional array transducer adapted for 3D fetal imaging; an image processor adapted to process image signals from the probe for display; an image display coupled to the image processor; a user control having a plurality of user-operable controls; a graphics generator responsive to a user control which identifies a region of interest (ROI) in an ultrasound image; a user control adapted to initiate processing of ultrasound data identified by the ROI to determine a fetal heart rate; an indicator responsive to the processing of ultrasound data to determine a fetal heart rate which indicates that a fetal heart rate has been determined; and an acquisition control adapted to be actuated by a user to acquire gated 3D fetal heart images when the indicator indicates that a fetal heart rate has been determined.