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Fetal stress during labour

17. What may stress the fetus during labour?

  1. Compression of the fetal head during contractions
  2. A decrease in the supply of oxygen to the fetus

18. How does head compression affect the fetus?

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.

Note
Head compression results in fetal vagal stimulation which slows the fetal heart rate during contractions.

19. What may reduce the supply of oxygen to the fetus?

  1. Uterine contractions: Uterine contractions are the commonest cause of a decrease in the oxygen supply to the fetus during labour, because there is reduced maternal blood flowing through the placenta even during a normal contraction.
  2. Chronically reduced blood flow through the placenta: The placenta may fail to provide the fetus with enough oxygen and nutrition due to a chronic decrease in the maternal blood flow through the placenta, i.e. placental insufficiency. Women with pre-eclampsia have poorly formed arteries that provide inadequate amounts of maternal blood to the placenta. Reduced placental blood flow can also be caused by narrowing of the uterine blood vessels due to maternal smoking or chronic hypertension.
  3. Abruptio placentae: With a placental abruption part or all of the placenta stops functioning because it is separated from the uterine wall by a retroplacental haemorrhage. In addition, the associated uterine spasm following a placental abruption severely reduces the maternal blood flow to those parts of the placenta which are still attached to the uterine wall. As a result, the fetus does not receive enough oxygen.
  4. Cord prolapse or compression: This stops the normal flow of blood from the placenta to the fetus and back again. As a result, the transport of oxygen from the placenta to the fetus is reduced.

Uterine contractions are the commonest cause of a decreased oxygen supply to the fetus during labour.

20. How do contractions reduce the supply of oxygen to the fetus?

Uterine contractions may:

  1. Reduce the maternal blood flow to the placenta due to the increase in intra-uterine pressure.
  2. Contractions of the uterine muscle compress the maternal vessels that supply blood to and from the placenta.
  3. Compress the umbilical cord.
Note
The maternal blood flow to the placenta depends on the maternal blood pressure, the resistance of the maternal blood vessels and the intra-uterine pressure. A fall in maternal blood pressure, increase in maternal vascular resistance (placental insufficiency) or increase in intra-uterine pressure will reduce the flow of maternal blood to the placenta.

21. How does the fetus obtain an adequate supply of oxygen during normal uterine contractions?

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.

22. When do uterine contractions reduce the supply of oxygen to the fetus?

Uterine contractions may reduce the oxygen supply to the fetus when:

  1. There is placental insufficiency.
  2. The contractions are prolonged
  3. The contractions are very frequent.
  4. There is compression of the umbilical cord.

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.

23. How does the fetus respond to a lack of oxygen during labour?

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.

24. How is fetal distress recognised during labour?

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.

Note
The fetus responds to hypoxia with bradycardia to conserve oxygen and protect the myocardium. In addition, blood is shunted away from less important organs, such as the gut and kidney, to essential organs, such as the brain and the heart. This may cause ischaemic damage to the gut and kidneys. The increased cerebral blood flow may result in intraventricular haemorrhage in the brain of preterm infants. Severe hypoxia will eventually cause a decreased cardiac output and hypotension leading to myocardial and cerebral ischaemia (not enough blood and oxygen supply to the heart and brain). Severe hypoxia also results in anaerobic metabolism which causes fetal acidosis (a low blood pH).