The pain of primiparous labour is said to be one of the most severe pains experienced, reported to be exceeded only by the pains of traumatic amputation and causalgia. Approximately 50% of women report severe or very severe pain during labour. Painless childbirth is a reality for only a small minority of women, although labour pain can be modified by a number of non-pharmacological manoeuvres.
The pain pathways involved in labour and delivery are extensive, involving afferent fibres from T10 down to S4.
Pain pathways
The uterus, lower uterine segment and cervix are all supplied by afferent AS and C fibres, which accompany the thoracolumbar and sacral sympathetic outflows. The pain of the first stage of labour is therefore referred to the dermatomes supplied by the same spinal cord segments that receive input from the uterus and cervix: T10-L1 during the first half of the first stage and then the lower lumbar and sacral dermatomes as labour progresses.
The second stage of labour may also involve somatic pain caused by distension and tearing of pelvic structures and by abnormal pressure on perineal skeletal musculature.
Modification of labour pain
Psychological factors
There is considerable evidence that preparation for childbirth can significantly modify the degree of pain experienced. This was the basis of the ‘childbirth without fear’ movement, which was popular in the 1960s. Although there can be little doubt that fear, fatigue and anxiety enhance pain perception, it is misleading for the majority of mothers to suggest that good antenatal education will lead to painless childbirth. Such expectations may in fact have the reverse effect, since the mother may develop complete loss of self-confidence when she begins to experience significant labour pain.
Women whose pregnancy is unplanned or unwanted are likely to experience more pain, as are those who have no birth partner to support them during labour. Conversely, the continuous presence of a midwife or female birth partner (doula) has been shown to reduce the amount of pain reported.
Cultural factors and ethnic group also have an influence on pain behaviour during labour, although it is likely that women from different cultures and of different racial groups actually experience similar levels of pain.
Promise of a finite duration of labour (as with active management of labour) may improve the ability to tolerate labour pain, although not necessarily reduce the level of pain experienced.
Physical factors
First labours are acknowledged to be more painful than subsequent labours, and older primiparae experience more painful labours than do younger women. Malpresentations, especially occipito-posterior positions, increase the pain of labour. Augmentation of labour by oxytocic drugs is reported to increase labour pain, and obstructed labour is more painful than normal labour. Tiredness is well known to reduce pain tolerance and is likely to occur if either the latent or the active phase of labour is prolonged.
There is a positive correlation between menstrual pain and labour pain, which has been postulated to be caused by excessive prostaglandin production.
Physiological factors
Progesterone may increase pain thresholds, and there is some evidence that in rats there is activation of endogenous opioid systems during late pregnancy. Experimental work in humans appears to confirm this.
Untreated pain causes an increase in circulating maternal catecholamines and other stress hormones. This may be detrimental to the mother with coexisting medical disease, and is also detrimental to the fetus. Maternal pain and acidosis are associated with reduced uteroplacental blood flow and fetal acidosis.
Key points
• The pain of childbirth is one of the most severe pains experienced.
• Pain from afferent AS and C fibres is referred to the dermatomes of T10 to S4 and is augmented during the second stage of labour by somatic pain from stretching and tearing of pelvic structures.
• Labour pain may be modified by antenatal education, and by the presence of a supportive partner.