M-mode, linear representation of adjacent cardiac structures motion as a function of time. A 2D relevant image of the fetal heart is first obtained; then, the M-mode cursor is placed at the targeted location on that image. In case of arrhythmia, the M-mode cursor is usually placed across an atrium and a ventricle, so that the relationship of atrial to ventricular contractions is recorded. Arrows indicate ventricular (V) and atrial (A) contractions.