1. A method for stabilizing a scan plane within a volume in medical diagnostic ultrasound imaging, the method comprising: (a) identifying a region of interest; (b) acquiring data representing at least portions of a three-dimensional volume positioned at least partly around the region of interest; (c) acquiring data representing sub-volumes of the three-dimensional volume, (c) using fewer scan lines than (b); (d) comparing the data representing the sub-volumes with the data representing at least the portions of the three-dimensional volume; (e) detecting motion of the region of interest within the three-dimensional volume as a function of (d); (f) positioning a two-dimensional scan plane within the three-dimensional volume as a function of the region of interest and the detected motion of the region of interest such that the two-dimensional scan plane is maintained at the region of interest in the volume despite the motion; and (g) acquiring a two-dimensional image responsive to the two-dimensional scan plane.