Psychology of Childbirth Notes

Learning Objectives

  • Understand the following:
    • The phenomenon of motor and sensory denervation in the term pregnant uterus.
    • Differences in denervation of the myometrium (muscle layer) and endometrium (lining).
    • The different stages of labour and their relationship to pain.
    • Possible alternative explanations for labour sensations and pain.
    • How increasing intensity of exertion can be experienced as pain.
    • The importance of vasculature and innervation during labour.
    • Function and vascular complexity of the placenta.
    • How labour pain serves to prevent injury to mother and baby.
  • Overall question: Why does labour hurt?

Denervation in the Pregnant Uterus

  • The uterus has a relatively low density of nerve fibers when compared to other smooth muscles in the body.
  • During pregnancy, the density further decreases due to:
    • Hypertrophy (growth) of uterine myocytes (muscle cells).
    • Decrease in the number of nerve fibers.

Changes in Uterine Nerve Fibres

  • Study findings (Tingåker et al. 2006) show:
    • Group 1: Non-pregnant uterus shows a higher count of nerve fiber profiles compared to:
    • Group 2: Term pregnant uterus,
    • Group 3: Uterus in labour shows the least amount of nerve fibers.

Motor Denervation

  • Orchestrated by ovarian hormones, leading to uterine quiescence.
  • Inhibition is relative; strong medications can induce abortion at any gestation stage.
  • Resistance to coordinated contractions remains, with hormonal control becoming predominant.

Structural Changes in the Uterus

  • Endometrium (Uterine Lining):
    • Contains glands and blood vessels that release hormones.
  • Myometrium (Muscle Layer):
    • Innervated by autonomic nerve fibers.
    • Experiences denervation during pregnancy.

Sensory Denervation

  • Not well understood; few studies conducted.
  • May reduce contractility risks since stretch sensations can induce contractions.
  • Stretch sensations from the myometrium are believed to be undetectable due to denervation.

Common Explanations for Labour Pain

  • Pain during labour typically arises from:
    • Stretching of the cervix
    • Contraction of uterine muscles
  • It's important to note that stretch receptors in the uterus and cervix disappear during pregnancy, complicating the explanation for pain:
    • These receptors are underdeveloped or absent at the onset of labour.
    • Cervical muscle fibers are largely replaced by collagenous tissue (extracellular matrix).

Recap

  • Key points:
    • Sensory and motor denervation of the myometrium occurs.
    • Stretch receptors disappear during pregnancy and at labour onset.
    • Denervation is a functional adaptation allowing the uterus to enlarge while preventing premature birth.

Stages of Labour

  1. Stage 1: Uterus contracts, stretching cervix to approximately 10 cm diameter.
  2. Stage 2: Baby passes through the birth canal and is born.
  3. Stage 3: Placenta is expelled.
  • Post-partum: Uterus contracts to regain its original form.

Cervical Changes During Labour

  • Stages include:
    • Not effaced and no dilation to full effacement and 1 cm dilation.
    • The longest and potentially most painful phase occurs during full dilation (10 cm).

Pain Characteristics During Labour

  • Pain during labour:
    • Felt primarily during contractions, with no pain in between.
    • Strongest sensations occur inside the body, particularly during active labour periods.
    • In cases of 'back labour,' pain continues between contractions with sensations felt at the lower back.