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Pathway of labour pain

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Pathway of Labour Pain

Labour pain is conducted through two distinct neurological routes that correspond to the two stages of labour.

Sources of Labour Pain

Pain arises from three main processes:
  1. Myometrial contractions against resistance of the cervix and perineum
  2. Progressive cervical dilation and stretching of the lower uterine segment
  3. Distension and compression of pelvic and perineal structures

First Stage of Labour - Visceral Pain Pathway

Origin: Uterine contractions + cervical dilation
Fiber type: Visceral afferent type C fibers (slow, unmyelinated)
Route of transmission:
  1. Afferent impulses from the uterus and cervix are picked up by C fibers
  2. These fibers travel with the sympathetic nerves (not independently)
  3. They pass through the uterovaginal plexus (Frankenhäuser's plexus)
  4. Then relay through the inferior hypogastric plexus
  5. Then through the middle and superior hypogastric plexuses
  6. Enter the spinal cord at T10-L1 nerve roots
Referred pain distribution:
  • Latent phase: T11-T12 dermatomes (lower abdomen, lumbosacral region)
  • Active phase: Expands to T10-L1 dermatomes (lower back, flanks, upper thighs)
This is why labour pain is referred to the lower abdomen and back - classic visceral referred pain.

Second Stage of Labour - Somatic Pain Pathway

Origin: Fetal descent + distension/stretching of the vaginal vault, pelvic floor, and perineum
Onset signal: The onset of perineal pain at the end of the first stage signals the beginning of fetal descent.
Fiber type: Somatic afferent fibers (faster, sharper, well-localized)
Route of transmission:
  1. Afferent impulses from the vagina, perineum, and pelvic floor
  2. Carried via the pudendal nerves (S2-S4)
  3. Enter the spinal cord at S2-S4
Pain distribution: T10-S4 dermatomes (entire lower abdomen, perineum, inner thighs, perianal region)

Summary Table

FeatureFirst StageSecond Stage
SourceUterine contractions, cervical dilationPerineal/vaginal distension, fetal descent
Pain typeVisceral (diffuse, cramping, referred)Somatic (sharp, well-localized)
Fiber typeC fibers (unmyelinated)Somatic afferents
Nerve routeSympathetic fibers → Uterovaginal plexus → Inferior hypogastric plexusPudendal nerve (S2-S4)
Spinal levelT10-L1S2-S4
Referred areaLower abdomen, back, flanksPerineum, inner thighs, rectum

Clinical Relevance

This anatomical pathway directly guides analgesic interventions:
  • Epidural analgesia for the first stage targets T10-L1 segments; coverage must be extended to T10-S4 for the second stage
  • Pudendal nerve block - local anaesthetic (e.g., 5 mL of 1% lidocaine) injected into the pudendal canal provides perineal analgesia for the second stage, forceps, and vacuum extraction
  • Paracervical block interrupts the pain pathway at the uterovaginal/paracervical plexus level (T10-L1), useful for first-stage pain
  • Spinal cord lesions above T10 prevent a woman from feeling any labour pain at all, because the afferent fibers from the uterus cannot reach consciousness
Sources: - Morgan and Mikhail's Clinical Anesthesiology, 7e, pp. 1606-1607 | - Barash, Cullen and Stoelting's Clinical Anesthesia, 9e, p. 3485 | - Textbook of Family Medicine, 9e, p. (OBSTETRIC ANESTHESIA section)
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