A prolonged deceleration of the fetal heart rate to 90/min indicates acute fetal hypoxia/acidosis, which is a decrease in the blood flow to the placenta that reduces the amount of oxygen and nutrients transferred to the fetus. Fetal pH drops at a rate of 0.01/min during the deceleration. Causes of acute hypoxia include low maternal blood pressure, dehydration, anemia, rapid uterine contractions, placental abruption, and fetal hypoxia.

fetal head compression causes early decelerations, which are benign and uniform in shape. They begin near the onset of a uterine contraction, and their lowest point occurs at the same time as the peak of the contraction.

fetal sleep cycle causes decreased variability of the fetal heart rate, which resembles a straight line. It does not cause prolonged decelerations.

fetal movement causes accelerations of the fetal heart rate, which are transient increases of at least 15 bpm above baseline for at least 15 seconds. They indicate fetal well-being and do not cause prolonged decelerations.