1. An ultrasonic diagnostic imaging system for identifying a fetal heartbeat, the system comprising: an ultrasound probe including an array transducer; an image processor adapted to process echo data from the probe for display; an image display coupled to the image processor and adapted to display an ultrasound image including a fetal heart; a graphics generator responsive to a user control that identifies a region of interest (ROI) in the ultrasound image; and a user control adapted to initiate generation of a plurality of spatially different M-mode lines associated with the region of interest, wherein the imaging system is adapted to (1) acquire echo data corresponding to the spatially different M-mode lines and (2) analyze the echo data to identify the fetal heartbeat associated with at least one of the spatially different M-mode lines, and wherein the system is adapted to rank the acquired echo data based on fetal heart rates measured for each of the corresponding M-mode lines.