This is an example of pain without illness. During pregnancy, the growing weight of the baby plus the uterus, placenta and amniotic fluid eventually add up to about 16 pounds (8 kilograms). This inevitably produces a change in posture, often with backache and sometimes nerve compression pain in the legs. For many months the baby has been floating free in the amniotic fluid and is felt to be kicking and sometimes somersaulting after about 16 weeks.
As the time of birth approaches for a normal delivery, the baby's head descends into the lower part of the uterus, a cone leading to the cervix, and becomes fixed. In abnormal births, the body and bottom sometimes fit into this space, and there is trouble ahead with a breech delivery as the body and legs are delivered first. At this time, the uterus begins to build up quite strong contractions that the mother can feel but without pain. The growing pressure bursts the membranes and the amniotic fluid drains out. The contractions now grow in frequency and intensity. The contractions of the uterus press directly on the baby's body and force its head into the cervix. The cervix is normally closed but the head forces it open until it is about 4 inches (10 cm) wide. The opening of the cervix is the first cause of pain, which the mother feels with each contraction, the pain centred in the lower abdomen, often extending in a band round the whole body from front to back and sometimes radiating down the legs. Between contractions there is a steady pain in the abdomen and back.
After this first stage of labour, the baby's head passes through the cervix into the vagina and begins to advance more rapidly. It presses on the vagina, bladder and rectum and then bulges out the floor of the pelvis, called the perineum. Since the location of the pressure has changed, the location of the pain shifts down to the pelvis and perineum. In order to speed up this particularly painful stage, forceps may be put on the baby's head and a cut, called an episiotomy, may be made to enlarge the opening of the vagina. After the delivery of the baby, the intensity of the pain drops dramatically. However, depending on the amount of inevitable tissue damage produced by the passage of the baby, prolonged inflammatory processes may now start up, producing long periods of soreness.
The sensory words most commonly used by mothers to describe the pain during labour are 'sharp', 'cramping', 'aching', 'throbbing', 'stabbing', 'hot', 'shooting', 'tight' and 'heavy'. For the emotional affective feelings, the most common words were 'tiring' and 'exhausting'. A rating scale for pain intensity was used with a number assigned by each mother. The average rating for women having a first baby was 35, and for those who had previous children the number was 30. These numbers are on a scale in which people with broken bones rate their pain at 20 and cancer patients at 27. Scores above 35 were reported only in cases of nerve injury or amputation. Evidently, the average reported pain of childbirth ranks high in the range of human experience. Of course, there is an extremely wide variation in the reported intensity and I have discussed variation elsewhere. But it is crucial to remember that, of first births in Canada, 9.2 per cent of mothers described their pains as 'mild', 29.5 per cent as 'moderate', 37.9 per cent as 'severe', and 23.4 per cent as 'excruciating'. A study of Scandinavian women (despite their reputation for toughness and stoicism) yielded similar results.
The cause of pain in the first stage of labour is the generation of nerve impulses in fine pressure-sensitive sensory fibres in the uterus and particularly the cervix. These impulses excite the spinal cord in the lower back, and all of the pain is referred to distant structures. Just as was described for the heart, the pain is not located in the uterus or cervix. As the baby's head advances into the vagina and presses on the pelvic floor, nerve impulses arrive in the sacral spinal cord and activate cells to produce a pain that is correctly located in the structures being squeezed.
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