Uterine Contraction Patterns and Risk of Harm
The fetus relies on 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 good oxygenation. Those patterns are discussed in another article on this website. 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 are a concern. Steps must be taken by nurses, midwives and doctors to avoid this occurrence.
There are many articles in the medical literature about the interpretation of fetal heart rate patterns in assessing fetal well-being, but 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.
Uterine Contraction Pattern
There are a number of characteristics of the uterine contraction pattern that may contribute to stress on the fetus during labour, thereby increasing the risk of harm for the baby. Uterine contractions can 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. Both events occur in most labours, without consequences for the baby. But where these events occur too frequently or too severely, the risk of injury increases.
There are a number of characteristics that demonstrate excessive uterine activity and must be avoided if injury to the fetus is to be prevented. Excessive uterine activity can lead to impaired oxygenation of the fetus. While the fetus has a remarkable ability to tolerate periods of impaired oxygenation, that tolerance has limits.
Eventually, if excessive uterine activity persists, the fetus can develop a condition called metabolic acidosis (also discussed in another article) and, if severe enough, brain injury and even death can occur. There are a number of contraction patterns that can contribute to the risk of injury or death to the fetus.
Contraction frequency is an important issue. The frequency of 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 a normal labour is one contraction every two to three minutes or fewer than five contractions in a 10 minute period. Where contractions occur more frequently, there is a risk that the fetus will not tolerate the added stress if this pattern is sustained. Overly frequent uterine contractions is called “tachysystole.”
In some labours a drug is used to assist with uterine activity. The drug is called “oxytocin” or “syntocinon.” The effect of this drug, administered to the mother, is to increase the frequency, duration and intensity of uterine contractions to promote delivery. One risk associated with oxytocin is the possibility of excessive uterine activity. When oxytocin is used, should overly frequent contractions occur (more often than one contraction every two minutes), the 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, delivery may be necessary at that point, particularly if the fetal heart rate pattern is non-reassuring.
The duration or length of contractions is another important feature. In a normal labour the desired length of contractions 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 of time, contributing to fetal stress.
A contraction that lasts longer than 90 seconds is called a “tetanic” contraction. Again, contractions lasting too long are abnormal and result in added stress on the fetus. These contractions must be avoided.
A Uterus At Rest
It is important for fetal well-being that the uterus rest between contractions. There are two important components to this rest: uterine resting tone, and uterine resting time. During contractions the uterus, which is 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 blood flow between contractions, which is a concern for fetal oxygenation. It is important for the nurse to touch the mother’s belly between contractions to ensure that the uterus is soft.
If there is persistent 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. Short resting times can contribute to unnecessary fetal stress.
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 is “tripling.” These patterns are abnormal and must be avoided.
Abnormal uterine activity should always be seen as 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.
A common feature in many of the cases that BILA lawyers handle for families of children born with cerebral palsy or other traumatic birth injuries is the presence of abnormal uterine activity, often associated with the injudicious use of oxytocin.
In many of these cases the injury suffered by the newborn would have been avoided entirely with the proper recognition of these patterns and the appropriate clinical response.
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