labor pain, nature of labour pain

Nature of labor pain: why it hurts and what to expect

Pain of childbirth ranks among the most intense pain experiences. Pain during first stage of labor becomes more intense as labor progresses. Labor pain is complex and is influenced by many factors.

Pain is a personal phenomenon. Although all of us have experienced pain at some time and know what it is, it is difficult to describe in words. In 1979 the International Association for the Study of Pain defined pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Pain has at least two components, sensory and affective (or emotional).

Sensory component is the physiological, mechanical. In other words it implies a straightforward anatomical and physiological structure: receptors sense the stimulus, the resulting nerve impulses are transmitted by the nerve fibers to the spinal cord and then to the brain where the sensation is consciously understood and interpreted as pain.

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Affective component implies some degree of distress associated with pain. It becomes more pronounced with the duration of pain episode. Patients having prolonged tooth ache describe it as exhausting, tiring, making them desperate. Affective components of pain is very prominent part of chronic pain conditions, such as chronic low back pain or migraines. Patients with these syndromes are often treated with antidepressant drugs, which target the affective component and improve overall pain control.

The experience of labor pain is a complex, subjective and multidimensional response to sensory stimuli generated during childbirth. Labor pain is a phenomenon embedded in the very nature of human existence and the relationships among us all. Unlike other acute and chronic pain experiences, labor pain is not associated with injury or damage but with the most basic and fundamental of life’s experiences: bringing forth of new life. Why this physiologic process should cause pain has been the subject of philosophic and religious debate. Scientists believe in a simpler biological explanation: labor pain is the Nature’s way to warn the expectant mother that labor has started, so that she can get to a safe place in which to give birth to her infant, as well as to seek the assistance of others for birth. Woman’s pain experience in labor is influenced by her physiology and psychology, as well as the culture she is coming from. That culture not only includes the beliefs, mores, and standards of her family and community, but also those of the health care system and its providers.

The current understanding of physiology of pain briefly may be described in the following terms. Sensory stimuli that signal peripheral tissue trauma are registered by the nerve endings and converted into electrical signals. These signals are then transmitted by the peripheral nerves to the spinal cord where they are processed and via specific pathways carried to the brain. Thalamus, brain stem and cerebellum are parts of the brain where decoding of pain stimuli occurs. In other words, in these regions primary physiological information related to pain originates in our consciousness: where the pain is located, how intense it is and so on.

Other parts, such as hypothalamus and limbic system, on the other hand, are areas which are responsible for emotional, or affective, responses. These areas are also responsible for so called autonomic, or involuntary reactions to pain: sweating, paleness, changes in pulse rate and blood pressure. At the level of the spinal cord and the brain neural pathways influence each other in such a way that the conduction of stimulus in one of them may be enhanced or inhibited by a stimulus in another. That’s why rubbing painful area reduces pain: nerve stimuli from the nerves that carry the sensation of rubbing inhibit the transmission of painful stimuli by other nerves. It is also believed that in simplistic terms these stimuli in a way compete with each other for the entry to the brain: rubbing taking some preference over pain, and pain perception is modified.

For practical purposes, clinicians define two main types of acute pain: somatic and visceral. Somatic pain originates in the superficial or close to superficial areas of the body: skin, muscles, ligaments, tendons and so on. It is typically well localized and of short duration after the damaging stimulus is removed. Typical example is pain from a pinprick: it is sharp, easy to locate and decreases rapidly.

Visceral pain is caused by processes in the internal organs, or viscera. Because of scarcity of pain receptors in the organs, this pain is poorly localized and is aching and of longer duration. Painful signals from the viscera often produce referred pain, which is felt away from the actual area of damage. Typical example of this is the pain in the left arm often reported by patients with heart attacks, or pain in the upper abdomen in patients with acute appendicitis.

During the first stage of labor the lower uterine segment and the cervix of the uterus are gradually becoming stretched. This process produces visceral pain. As the first stage of labor progresses the intensity of pain grows. The nerves at the level from T10 to L1 transmit the labor pain during this stage, that is why most pain is felt in the lower abdomen. Women also often report referred pain in the back, buttocks, hips and the actual wall of the abdomen. Back pain is the most common location of referred pain and is experienced by up to 74% of women in labor.

The exact experience of pain during this stage varies, and some patients describe it as widespread, poorly localized, whereas others feel it in specific areas. As the labor progresses and reaches late first or early second stages and the fetus begins descending, the pain becomes somatic. It is sharp in nature, always well localized and is transmitted by pudendal nerves that originate in sacral region on both sides.

Pain in labor triggers the complex cascade of physiological processes typical for fight-or-flight reaction. Because of adrenaline release blood pressure and pulse rate increase. In patients with pre-existing heart conditions this may lead to problems. Rate of breathing also increasing, and hyperventilation leads to decreased levels of carbon dioxide in the blood. This causes constriction of blood vessels supplying the brain and may cause lightheadedness or even fainting. Most obstetric patients are young and relatively fit, and these physiological responses do not lead to problems.

The first step to treatment of pain is assessment of its intensity. The assessment of labor pain is complicated. As pain is a subjective experience the only measurement available to us is patient’s report. In clinical practice pain is measured by questionnaires. For acute pain several pain scales have been developed. For instance, the patient may be asked to rate his or her pain on the scale from one to ten, ten being the most intense pain ever experienced by the patient. Or as mild, moderate or severe. For assessment of acute – post-traumatic or post-surgical pain these scales work well. However simple grading is not adequate for measuring more complex painful conditions. Pain may be burning or breaking, pulsating or constant. It can be exhausting and tiring and so on.

In order to assess the complexity of pain in patients with chronic pain syndrome McGill Pain Questionnaire has been developed. It consists of several groups of questions each targeting separate features of pain.

In 1981 Ronald Melzack, a renowned researcher in the area of pain, used McGill Questionnaire to evaluate pain in laboring women. A total of 141 women were studied, 91 of them received prepared childbirth training in the months before. It was evident that labor was significantly more painful for the first birth than for later births. When compared the intensity of labor pain with that of other pain syndromes obtained in earlier study, the authors noted that labor pain ranks among the most intense pains recorded with their questionnaire. While the average intensity of labor pain was extremely high, there was a wide range in pain scores. Twenty five percent (25%) of the primiparas and nine percent (9%) of the multiparas reported their pain as horrible or excruciating; twenty three (23%) of the primiparas and eleven percent (11%) of the multiparas had pain scores in the top third of the range. In other words, almost half of those giving birth for the first time and one in five of mothers with previous childbirth experience had very severe pain.

Describing the quality of their pain more than half of women chose words such as “sharp, cramping, tiring (49%) and intense”. Of note, there was small reduction in some qualities of pain among women who underwent childbirth training, but no differences in requests for epidurals.

Similar study conducted in 1884 confirmed these findings. Women were also assessed with McGill questionnaire, this time at different stages of labor. Levels of pain varied greatly between subjects, but on an average were found to be severe.

The affective, or emotional, component of pain seems to be greater throughout the first stage of labor for nulliparous as compared with multiparous parturients, but it tends to decrease in both groups during the second stage.

There were no further studies evaluating labor pain, probably because there is no need to study the obvious: labor is painful.

Pain is a private experience and is best measured by those who feel it, the patients. However, health care workers, the members of the family and simply bystanders often overestimate their ability to asses someone else’s pain, most often based on the behavior. In some situation such assessment is useful. For example, specially developed protocols are used to measure the pain experienced by small children who are not able to communicate, by observing their behavior: crying, movement, facial expression and other signs.

Several studies used the Present Behavioral Intensity (PBI) protocol in laboring women. PBI protocol is based on observing various behavioral signs – breathing pattern, movement and agitation – in order to get an idea of the patient’s pain. Invariably the observers – medical staff, midwives, doctors or members of the family – underestimate the severity of woman’s pain. Neither there is correlation of how much pain the woman in labor feels and the assessment of the observers at any given moment. In other words, most of the time nobody can guess how much pain the woman is experiencing. Recent research has also suggested that the cultural gap between medical personnel and patients also affects the care provider’s interpretation of the woman’s pain experience, the wider the gap the less reliable this interpretation becomes.

Generally, the intensity of labor pain increases as the cervix of the uterus dilates. It also depends on the intensity, duration, and frequency of contractions of the uterus. However, each woman’s experience varies widely, and pain during labor may fluctuate up and down significantly.

The pattern of pain during labor appears to be somewhat different in nulliparous as compared with multiparous women. Consistent findings indicate that during early labor (before 5 cm), nulliparous women on average experience greater sensory pain than multiparous women. As labor progresses these differences become less pronounced. This observation may be explained by the physiologic differences between nulliparous and multiparous women. Most of the pain during the first stage of labor is produced by the stretching of the lower segment of the uterus and the cervix. Women who previously gave birth have suppler, more distensible tissues, therefore feel less pain. As labor progresses, the fetus descends through the pelvis faster, and this suddenness causes more pain in these patients. In women giving birth for the first time, on the other hand, fetus descends slower and this causes less pain during the second stage.

Previous experience with pain may also affect the intensity of pain in labor. A study from Britain used 97 women in labor for the assessment of pain. Ten women were identified who reported that they had never experienced pain of any type before giving childbirth. These women experienced significantly less sensory, affective, and total pain during labor than the rest of the women, who had previously experienced some pain outside of childbirth. Since there were no other identifiable differences between the two groups of women, it was suggested that those who never experienced pain might be physiologically relatively insensitive to it.

On the other hand, another study on labor pain intensity found the opposite: previous experience of pain was strongly associated with perceived levels of pain in labor. Women who reported that they had previously experienced significant levels of pain unrelated to childbirth had low or moderate levels of labor pain. Subjects who reported little experience of pain unrelated to childbirth, had high levels of pain in labor. It seems logical that previous pain experience provides the opportunity to develop pain-coping skills and more accepting attitude towards future painful experiences. Because for women in developed countries childbirth is the first really painful experience, this may partly explain why giving birth for the first time is often more painful than during consequent labors.

Many cultural, ethnic and educational factors also influence woman’s experience of pain in labor. Some studies found no differences in the intensity of pain between women from different countries or ethnic groups, namely, African American and white American, Australian and Italian-born, Dutch and American, Kuwaiti, Bedouin and Palestinian, or between Jewish and Bedouin women. The findings of others, however, suggest that pain behaviors may vary greatly among different cultural groups as a result of learned patterns of expected behavior. This is well illustrated in a study analyzing 194 American and 152 Dutch women in two university hospitals. 61% of the Dutch women received no pain medication during labor, versus only 16% of the American women, though their expectations of labor pain were about the same. The American women expected labor to be more painful and expected to require more medication to manage the pain of labor. Both groups of women (79.9% American and 84% Dutch) preferred the same method of pain management as they had just experienced for a subsequent labor, a finding that suggests both groups were satisfied with their experience despite wide variation in the use of medical intervention for pain. This single comparative study highlights the important influence of culture on expectations and attitudes toward labor pain. According to the authors, the Dutch see birth as a natural process and are biased against any sort of interference, and are more inclined to let the nature take its course.

Ethnicity is another factor that may influence pain experience. A study from Israel found Western Israeli women reported significantly less pain during labor than Middle-Eastern Israeli women. At the same time among the latter, those with lower education level reported significantly more pain than women of high education. This suggested that education can reduce the influence of culturally learned attitudes and expectations of labor pain.

Not surprisingly, anxiety is another factor that may increase pain, generally and during labor as well. Excessive anxiety triggers “fight-or-flight” response which, in turn, may amplify painful stimuli from the uterus and the cervix and make pain more intense. Fear of pain may be one component of labor-related anxiety and has a high correlation with pain levels reported during first-stage labor.

One study conducted in 1990 tried to determine whether women’s attitudes and concerns, confidence in ability to control pain, and practice of pain-control techniques would predict pain and coping or distress-related thought during labor. During the third trimester of their pregnancies, 115 women completed the prenatal self-evaluation questionnaires that assessed their confidence and practice of pain-control techniques. During the latent (cervical dilatation less than or equal to 3 cm), active (4-7 cm), and transition (greater than or equal to 7 cm) phases of labor, women were asked questions in relation to their levels of pain, as well as their thinking pattern that ranged from constructive, coping-related thought to distress-related thought.

Women’s confidence in their ability to use relaxation techniques and their reported practice of pain-control strategies did predict lower levels of pain and greater coping-related thought during latent labor, but not in active and transition phases. Fear of pain and feeling of helplessness before childbirth predicted high levels of distress during latent phase of labor. Concern for self and baby were predictors for distress and pain during active and transitional phases. The authors suggested that as labor progresses into the active phase, women’s fundamental concerns and anxieties push away the skills acquired through childbirth education. This has been described earlier: the title of the study mentioned earlier in the text, the one where McGill Questionnaire was used to assess labor pain, was “Labor is still painful after prepared childbirth training”…

Some studies claim success in using various pain-coping techniques for reducing labor pain. Categories of strategies described by women include relaxation, distraction, imagery, reversal of affect, breathing techniques, normalization, control, idiosyncratic strategies, focusing and so on. But to date, there is little evidence to associate attendance at antenatal classes with a reduction in psychological distress or increased satisfaction with the experience of labor.

Woman’s confidence in her ability to be able to cope with the distress of childbirth has also been a strong predictor in the intensity of labor pain. When various factors that may influence labor pain were analyzed – fear of pain, confidence, concern about the outcome of labor, prior experience with non-gynaecologic pain, cervical dilatation, frequency of contractions, menstrual pain, pregnancy, weight/height ratio, and fetal weight – confidence has consistently emerged as the most significant predictor of first-stage labor pain, explaining overall 30% of the variance in pain.

The environment affects the woman’s experience of pain. Environment includes the persons present and their verbal and nonverbal communications; the philosophy of care and practice policies of the providers; the quality of support the woman perceives from those present; the degree of strangeness of the environment, including the furniture and equipment that make up the environment; noise, lighting, and temperature; and the restrictiveness of the environment in terms of space or movement with the space.

In a study conducted in Denmark, pethidine was administered 4 times more frequently to low-risk parturients in hospital than to similar parturients who chose birth center care. Although there were no significant differences between the two groups of women in regards to number of previous childbirth, marital status, or childbirth education, the women who delivered at the birth center were significantly older and of a higher social group. Interesting that among 41 women who had planned a birth center delivery but were sent to the hospital because the birth center was full, the rate of pain medication administration was identical to that of the women with planned hospital births. This suggests that the environment itself, with its specific care approaches, may have affected the women’s ability to cope with pain and was reflected in an increased request for pain medication.

Another trial in Sweden compared 617 women who gave birth in birth center with 613 women who received standard in-hospital care. After labor, women in both groups had similar memories of the intensity of their pain, though women receiving standard care used significantly more pharmacologic pain relief (epidural, pethidine, nitrous oxide, pudendal block) than was used by the women in the birth center. Although multiparous birth-center-care women reported significantly higher pain intensity than their hospital-care counterparts, they were no less satisfied with the quality of the birth experience, their own achievement in childbirth, their involvement in the process, or their sense of anxiety during labor than the multiparas who delivered in the hospital.

The pain of childbirth is a complex phenomenon which is influenced by a multitude of factors, often in contradictory manner. However the fact that labor is accompanied by considerable – if not very severe – pain does not require proof.

References:

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17. Waldenström U, Nilsson CA. Experience of childbirth in birth center care. A randomized controlled study. Acta Obstet Gynecol Scand 1994;73:547-54.

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Dr. Eugene Smetannikov is a practicing anesthesiologist with the interest in obstetric anesthesia. He is the author of the most comprehensive book on the subject, The Truth About Labor Epidural