An ultrasonic diagnostic imaging system acquires 3D data sets of the fetal heart by use of a gating signal synthesized from detected motion of the fetal heart. A sequence of temporally different echo signals are acquired from a location in the anatomy where motion representative of the heart cycle is to be estimated, such as a sample volume in the fetal carotid artery or an M line through the fetal myocardium. A heart cycle signal is synthesized from the detected motion and used to gate the acquisition of fetal heart image data at one or more desired phases of the fetal heart cycle. In an illustrated embodiment 3D data sets are acquired from multiple subvolumes, each over the full fetal heart cycle, then combined to produce a live 3D loop of the beating fetal heart.