Late decelerations are a symmetric fall in the fetal heart rate, beginning at or after the peak of the uterine contraction and returning to baseline only after the contraction has ended. Late decelerations are associated with uteroplacental insufficiency.
Variable decelerations show an acute fall in the FHR with a rapid downslope and a variable recovery phase. They are characteristically variable in duration, intensity, and timing, and may not bear a constant relationship to uterine contractions. Variable decelerations are due to cord compression.
Early decelerations are physiologic caused by fetal head compression during uterine contraction, resulting in vagal stimulation and slowing of the heart rate. This type of deceleration has a uniform shape, with a slow onset that coincides with the start of the contraction and a slow return to the baseline that coincides with the end of the contraction. Thus, it has the characteristic mirror image of the contraction.
The true sinusoidal pattern is a regular, smooth, undulating form typical of a sine wave that occurs with a frequency of two to five cycles/minute and an amplitude range of five to 15 beats/minute. It is also characterized by a stable baseline heart rate of 120 to 160 beats/minute and absent beat-to-beat variability.
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