This definition emphasises that like hunger, fear, envy, or happiness, pain is fundamentally an emotional experience. Consequently, the experience of pain cannot be measured directly, nor can it be felt by anyone other than the sufferer. We should resist the temptation, therefore, to try and judge other people's pain. It is sometimes said, for example, that an individual has a low, or a high, 'threshold' to pain. Statements such as these are not only meaningless, but unfair and rather self-righteous.

In this section we will explore the nature of labour pain according to the pain 'pathway' that is followed by the painful stimulus and will show how various techniques can be used to 'target' the pain at different anatomical sites in this pathway.

The source and origin of childbirth pain

Labour is associated with two different kinds of pain. The first arises from the uterine muscle when it begins to contract at the onset of labour. Because the uterus is an internal organ, the pain associated with contractions is called visceral pain (from viscera, meaning internal organ). Like all sources of visceral pain (e.g.intestinal colic, gallstones, appendicitis), pain arising from the uterus cannot be accurately pin-pointed.

Visceral pain is also often felt in a different part of the body than from where it originated. This is called referred pain. In the case of the uterus, contraction pain is commonly referred to the lower back and sacrum; indeed, some women find that their labour feels more 'painful in the back' than in the abdomen. In other words, uterine contractions are typically felt over a large area of the body: namely the lower abdomen, small of the back, upper thighs and bowel. This distribution reflects the nerve supply to the uterus.

Uterine contraction pain is initially dull and aching in character. As the contractions become stronger, so does the pain intensity. The interval in between contractions is usually free from pain. Pain referred to the lower back, however, is often felt in between, as well as during, contractions. There is some evidence that backache during labour is more common among women who experience backache during menstruation.

Contraction pain is caused by uterine distension and stretching of the cervix. Surprisingly, the uterus can be cut or incised without causing pain. Whether contraction pain is mediated by the direct stimulation of nerve endings sited within the uterus; the release of locally produced irritant substances or ischaemia (insufficient blood flow, similar to the pain of a 'heart attack'), is not entirely clear. Several chemical compounds known to produce severe pain have been isolated from tissues that have been injured. They include such exotic sounding names as histamine, 5-hydroxytryptamine, bradykinin, substance P and prostaglandin.

Whatever the actual mechanism may be, the lower part of the uterus has a rich nerve supply and it is these nerve endings which are stimulated to initiate the sensation of pain during a contraction. Contraction pain can also be caused by pressure of the uterus on surrounding structures within the pelvis, such as the bladder, rectum and nerve trunks going to the lower limbs. (Occasionally, the sciatic nerve itself is stretched - causing pain to radiate into the back of the thighs and legs).

The second kind of pain occurs closer to delivery. Unlike visceral pain, it is sharp or stinging in character and very accurately localised to the vagina, rectum and perineum. This type of pain is called somatic pain (from soma, meaning body surface) and is due to stretching of the structures of the lower birth canal. It is caused by the descent of the fetus and is later accompanied by a very powerful urge to push, or bear down.