Physiology of pain in labour.

Labour pain is the result of many complex interactions. Although not fully determined, the pain arises from distension of the lower uterine segment and cervical dilatation. The neural mechanism of labour has some features similar to other forms of acute pain; nociceptive information is relayed in small A delta and C afferent fibres to the dorsal horn of the spinal cord, mediated by neurotransmitters; from there it may be involved in the initiation of segmental spinal reflexes or pass through the spinothalamic tract to the brain. Many factors are activated during labour which may modify the nociceptive impulse at different stages of its passage. Some of these factors act synergistically to promote anti-nociception that peaks at delivery.