The fetus relies on the adequate circulation of blood in the uterus and umbilical cord to provide enough oxygen to maintain fetal well-being during labour. The condition of the fetus is assessed during labour by monitoring the fetal heart rate. There are characteristic patterns of the fetal heart rate reflected on a tracing that indicates proper oxygenation. Those patterns will be discussed in a later article.
This article is concerned with the effect of uterine activity on fetal well-being. The fetus is at risk for impaired oxygen delivery where the uterine contraction pattern is abnormal. An abnormal uterine contraction pattern accompanied by non-reassuring fetal heart rate patterns is concerning. Nurses, midwives, and doctors must take steps to avoid this occurrence. The importance of abnormal uterine activity as a contributor to fetal stress during labour is, in my view, not adequately recognized in much of the literature on the management of labour and delivery. Much of the literature fails to address or adequately consider the uterine contraction pattern as a critically important variable in the evaluation of fetal well-being.
Recent Guidelines published by the Society for Obstetricians and Gynecologists of Canada (SOGC)
Recently published SOGC Guideline, No. 396 published in March 2020, attempts to categorize all abnormal uterine activity under one name – "tachysystole." That term, however, means rapid contractions. I do not think it is a helpful descriptor for all the troublesome contraction patterns that nurses and doctors need to be concerned with. I address this new Guideline from the SOGC in a later publication. Abnormal uterine contraction patterns may contribute to stress on the fetus during labour, thereby increasing the risk of harm for the baby.
Oxytocin, a medication given to some women in labour to increase the intensity and frequency of uterine contractions, can increase these stressful contractions. Uterine contractions may compress the blood vessels in the uterus, potentially interfering in the transfer of oxygen to the placenta and the baby. Contractions can also compress the umbilical cord, which may affect the flow of oxygenated blood to the baby. Although every labour has some degree of stress on blood flow, where these stressors are abnormal, there can be adverse consequences for the baby. Without interventions to relieve undue stress, the baby is at risk of injury to the brain from insufficient oxygenation.
A fetus has a remarkable ability to tolerate periods of impaired oxygenation, but that tolerance has limits.
Eventually, if excessive uterine activity persists, the fetus can develop a condition called metabolic acidosis (also discussed in another article). If severe enough, brain injury, and even death, can occur. There are some contraction patterns that can contribute to the fetus's risk of injury or death. Contraction frequency must not be excessive. Uterine contractions must be sufficient to dilate the mother's cervix and promote the descent of the fetus down the birth canal.
Generally speaking, the desired frequency of uterine contractions in normal labour is one contraction every 2 to 3 minutes or fewer than 5 contractions in 10 minutes. Where contractions occur more frequently, there is a risk that the fetus will not tolerate the added stress if this pattern is sustained. Oxytocin increases the frequency, duration and intensity of uterine contractions to promote delivery.
Not every mother's uterus responds favourably to this drug. In some cases, excessive amounts of oxytocin are used. Where overly frequent contractions occur (more often than one contraction every 2 minutes), oxytocin must either be turned down or stopped. The abnormal uterine contraction pattern must be observed to see if it resolves. If stopping the oxytocin does not fix the problem, the delivery may be necessary at that point, particularly if the fetal heart rate pattern is non-reassuring.
The duration or length of contractions is also important. The desired length of a contraction is between 45 and 60 seconds. Contractions that last longer than 60 seconds, if persistent, may indicate that the uterus is contracted for excessive periods, contributing to fetal stress. A contraction that lasts longer than 90 seconds is called a "tetanic" contraction. Contractions lasting too long are abnormal and result in added stress on the fetus.
It is crucial for fetal well-being that the uterus rest between contractions.
There are two essential components to rest: uterine resting tone; and uterine resting time. During contractions the uterus, a muscle, tenses to provide the force needed to advance the labour. The uterus is taut during contractions or said to have increased "tone." Rest between contractions requires the uterus to be "soft" when touched or palpated. If the uterus is not soft, then the tone is increased. The muscle may not be sufficiently relaxed to promote good circulation of blood. This could result in decreased flow between contractions, which is a concern for fetal oxygenation.
The nurse needs to touch the mother's belly between contractions to ensure that the uterus is soft. If there is a persistently increased tone between contractions, the cause for this problem must be identified and a plan to manage the problem established, which may involve decreased oxytocin or expedited delivery. The resting time between contractions is also important. There must be sufficient time between contractions to allow the fetus to "recover" from the stress of the previous contraction. Ideally, the resting time between contractions should be one minute.
Contractions that occur in quick succession may also be problematic.
Where two contractions occur very quickly with little rest in between, it is called "coupling." Three contractions in quick succession are "tripling." Abnormal uterine activity is concerning whether the patterns are seen with or without non-reassuring fetal heart rate patterns. If excessive uterine activity occurs with a non-reassuring fetal heart rate pattern, it is that much more worrisome and must be addressed clinically. Doctors and nurses must be sensitive to the possibility of a building fetal metabolic acidosis when these conditions occur together and are required to take action.
Many of our birth injury cases involve excessive uterine activity with an excessive infusion of oxytocin. In many cases, the injury could be avoided by discontinuing oxytocin in the face of these abnormal patterns and reconsidering whether it remains feasible to anticipate a "safe" vaginal delivery.
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