An early deceleration and a late deceleration may visually appear identical. Both are smooth and curvilinear and appear to be a mirror image of the contraction. The distinction between the two is based upon the relationship of the deceleration to the uterine contraction (UC).
  
Early decelerations correspond, temporally, to the contraction and therefore exist only as a periodic change. Early decelerations are a benign finding caused by a vasovagal response as a result of fetal head compression by the contraction. Pressure on the fetal skull alters the cerebral blood flow and this in turn stimulates the vagus nerve. The heart rate is gradually decreased as the pressure of the contraction intensifies, and the deceleration gradually resolves as the pressure resolves. This pattern is generally limited to the active stage of labor. If the pattern is found in early labor, it may be associated with cephalopelvic disproportion (CPD).
  
Early decelerations have not been associated with fetal hypoxemia or acidosis.

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

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