"Labor pain is among the most severe pain human beings experience and compares in its intensity to severe cancer pain or pain from the amputation of a digit. Parity also influences labour pain and primiparous woman experience more pain during early labour, while multiparae have greater pain in the second stage. Although several factors influence parturition pain and its severity varies widely, the occurrence of truly painless labor is rare" (1) (Figure 58.1).

Labour pain is described as intolerable in about one third of women (2) (Chapter 64). The nature of parturitional pain (incidence, intensity and quality) is extensively described in a recent edited book (3).

Pain in the first stage of labour results from dilatation of the cervix and lower uterine segment and from distension of the body of the uterus by uterine contractions and is transmitted by afferent nerve fibers (accompanied by sympathetic fibers) of the 10th, 11th and 12th thoracic and 1st lumbar nerves. The descending head causes pressure on the lumbosacral plexus responsible for pain felt in the back , thighs and legs (Figure 46.1).

Pain in the second stage of labour results from distension of the birth canal and perineum and is transmitted by the 2nd, 3rd and 4th sacral nerves (Figure 46.1).

The degree of discomfort is described in the following table (shortened and modified) from Bonica (4) (Table 58.1).

References:
1. Writer WDR Physiology of Labor. In: Handbook of obstetric analgesia and anesthesia. (Eds McMorland G, Marx F). WFSA p24 - 34, 1992

2. Melzack R. The myth of painless childbirth. Pain 1984;19:321-325.

3. Pain relief and anesthesia in obstetrics. Eds: van Zundert A, Ostheimer GW. 1995. Churchill Livingstone, New York. p19-52

4. Bonica JJ. Obstetric analgesia and anesthesia. Springer-Verlag Berlin, Heidelberg, New York 1972. p 31-33.