Variable decelerations appear morphologically different from either early or late decelerations. They are characterized by a sudden, abrupt drop in the FHT along with usually a similarly abrupt return to baseline. The shape varies widely, and they may appear as a V, U, or W. They may be either periodic or episodic. Often, there are small accelerations immediately before and immediately following the deceleration. These associated accelerations are often referred to as shoulders.
   
   The classic mechanism described as the cause of variable decelerations is umbilical cord compression. Initial or mild umbilical cord compression results in occlusion of the umbilical vein, which is larger than the arteries and less rigid. This results in decreased venous return resulting in reflex tachycardia to maintain cardiac output. This explains the initial increase in heart rate (shoulder) preceding the deceleration. Further compression of the cord leads to occlusion of the umbilical artery, and the resulting increased systemic resistance, sensed by the baroreceptors, results in a protective reflex slowing of the heart rate. As the cord is decompressed, this series of events is reversed, and an acceleration may follow the deceleration (artery is decompressed but the vein is still compressed) prior to returning to baseline.
  
   Variable decelerations are classified as severe when they last more than 60 seconds, fall below 70 beats/min, or have a drop of 60 beats/min below the baseline rate.
  
   While umbilical cord compression is typically responsible for this pattern in the first stage of labor, it may also result from head compression during the second stage of labor.
  
   Variable decelerations may be seen in the antepartum or early in labor if associated with oligohydramnios. Similarly, they may be detected after rupture of membranes, and in this setting they may portend cord prolapse, particularly if rupture of membranes occurred prior to engagement of the presenting part. During the descent phase of labors, typically between 8-10 cm cervical dilation, variables may be associated with nuchal cords causing cord stretch or compression, or with head compression associated with rapid descent and maternal valsalva.
  
   Variable decelerations in the presence of normal FHR variability are not thought to represent hypoxia, but repetitive severe variable decelerations with diminished or absent FHR variability may indicate hypoxia.

See illustration below for an example of variable deceleration. Please click to enlarge.

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