Late Deceleration
A late deceleration is a smooth, gradual, symmetrical decrease in fetal heart rate beginning at or after the contraction peak and returning to baseline only after the contraction has ended25.
A gradual decrease is defined as 30 seconds or more from the onset of the deceleration to the nadir.
In late deceleration, the onset, nadir and recovery of the deceleration occur after the beginning, peak and ending of the contraction, respectively25 .
The depth of late decelerations is not more than 30 to 40 bpm below baseline and not more than 10 to 20 bpm. Late decelerations usually are not accompanied by accelerations.
The interval or lag period from the onset of contaction till the onset of late deceleration is related directly to basal fetal
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Acute late decelerations may be caused by placental abruption25.
Variable Deceleration
These are most common deceleration patterns encountered during labor. Variable deceleration is defined as an abrupt decrease in the fetal heart rate beginning with the onset of the contraction and reaching a nadir in less than 30 seconds25. The decrease in fetal heart rate lasts between ≥ 15 seconds and 2 minutes and is ≥ 15 bpm in amplitude. The onset of deceleration varies with successive contractions. Cause of variable deceleration is attributed to umbilical cord occlusion25 .
When there is interruption of umbilical blood flow or changes in placental bed oxygenation, changes occur in fetal blood pressure which induces fetal heart rate reflexes causing variable deceleration. According to the American College of Obstetricians and Gynecologists11, recurrent variable decelerations with minimal to moderate variability are indeterminate, whereas those with absent variability are