Pain in labour is transmitted along the A - d (delta) and C nerve fibres that are designed to sense sharp, burning or aching pain. These nerves sense the contractions of the uterus, the dilation of the cervix and the pressure and stretching of the vagina, pelvic floor and perineum. The A - b (beta) skin nerve fibres come into play in the final stages of the birth, as the skin of the perineum and anus stretch.
The changes in the uterus, cervix and vagina often refer pain onto other areas in the body supplied by these types of nerves, such as the woman's thighs, hips, back, belly, anus and rectum. Sometimes these areas sense a constant, milder pain that lingers in between the more intense contraction pains.
Image 8-01 shows how the nerve fibres travel from the spinal cord to the uterus, cervix and vaginal areas.
Prelabour and early 1st phase of labour
Active phase of 1st stage
End of 1st stage or transition
The 2nd stage or pushing phase
Prelabour and early 1st phase of labour
As the labour begins, the pain messages are sent along the woman's nerve fibres to her spinal cord. Here they meet and relay the pain messages to the ascending nerve fibres in the spinal cord, to be taken up to the woman's brain and interpreted as a conscious pain sensation.
If the larger A - b (beta) skin nerve fibres are stimulated at this time with touch, heat, cold or massage, they should intercept the pain messages by reaching the spinal cord first, effectively shutting the 'gate' to lessen the interpretation of the pain for the woman.
The final physical pain message that eventually reaches the woman's brain is individually modified for interpretation by her and expressed as a reaction or response to the pain she is experiencing. The level of reaction or response will be influenced by the woman's personal beliefs, cultural background, ideas, expectations, fears and feelings about her pain and the environment she is in.
Active phase of 1st stage