Acceleration of FHR can also be induced by vibroacoustic stimulation using an external source of sound such as an artificial larynx applied to the maternal abdomen over the fetal head. Deviations from the baseline occur in response to uterine contractions and fetal activity that affect fetal oxygenation and transfer of carbon dioxide. Accelerations are transient increases of the fetal heart rate, occurring at the same time as uterine contractions and coming at anytime during labor. Decelerations are decreases in the fetal heart rate. The three types are early, late, and variable.
Early deceleration is believed to be caused by fetal head compression.
The fetal heart rate returns to the baseline at or before the end of the uterine contraction and is not associated with an abnormality. Late deceleration occurs after the uterus has begun to contract and may not cease with the contraction. It is an ominous sign of fetal distress, is believed to be caused by uteroplacental insufficiency, and is frequently associated with high-risk pregnancy and with maternal hypotension or uterine hyperactivity. Variable deceleration occurs at various times in relation to uterine contractions. It could be associated with umbilical cord compression and may be relieved by changing the mother's position.