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During uterine contractions, especially during very strong contractions close to delivery, the fetal head is squeezed. This may result in a slowing of the fetal heart rate (a deceleration) during the middle of a contraction, when the pressure in the uterus is highest. This slowing of the fetal heart rate during the middle of a contraction is called an early deceleration. Head compression during normal uterine contractions may also result in early decelerations, but usually does not harm the fetus.
Early decelerations are caused by head compression.
Uterine contractions are the commonest cause of a decreased oxygen supply to the fetus during labour.
Uterine contractions may:
There is very little maternal blood flow through the placenta during the peak of a normal uterine contraction. However, there should be enough oxygen stored in the pool of maternal blood in a healthy placenta during a contraction to meet the needs of the fetus throughout that contraction.
Between contractions fresh maternal blood, loaded with oxygen, flows into the placenta. Therefore, normal contractions in labour usually do not affect the oxygen supply to a healthy fetus with a normally functioning placenta.
Uterine contractions may reduce the oxygen supply to the fetus when:
Frequent and prolonged uterine contractions do not allow enough time between contractions for the maternal blood in the placenta to be replaced by fresh maternal blood loaded with oxygen.
A reduction in the normal supply of oxygen to the fetus causes fetal hypoxia. This is a lack of oxygen in the cells of the fetus. If the hypoxia is mild the fetus will be able to compensate and not become distressed. However, moderate or severe fetal hypoxia will result in fetal distress. Severe, prolonged hypoxia may eventually result in fetal brain damage or even fetal death.
Severe fetal hypoxia results in fetal distress.
Fetal distress, caused by a lack of oxygen, results in a slowing of the fetal heart rate. Unlike early decelerations, when the fetal heart rate slows during head compression, hypoxia causes the fetal heart rate to slow towards the end of the contraction. The slow fetal heart rate only speeds up again after the contraction has ended. This is known as a late deceleration. Therefore, late decelerations are due to fetal hypoxia.
During a late deceleration, the fetal heart rate is slowest at the end of, and immediately after, the contraction, because this is when there is the least amount of oxygen in the placenta.
Late decelerations occur towards the end of a contraction and are caused by hypoxia.