Physiology of Labour

Overview

• Labour involves coordinated uterine contractions leading to cervical effacement and dilation, followed by birth.
• Divided into four stages:
  1. Effacement and dilation of cervix.
  2. Birth of the baby.
  3. Birth of the placenta.
  4. Recovery and establishment of uterine tone post-birth.
• Gradual transition between pregnancy and labour; overlapping physiological changes.

Uterine Changes Pre-labour

• Myometrium (non-striated muscle) prepares by increasing bulk via hypertrophy and hyperplasia.
• Contractions during pregnancy are mild and irregular due to inhibitors (progesterone, nitric oxide).
• Activation of myometrium includes:
  • Hormonal ratio changes.
  • Increased electrical activity.
  • Enhanced myometrial cell responsiveness.
  • Increase in ion channels.

Initiation of Labour

• Complex process involving maternal and fetal mechanisms.
• Hormonal changes initiate labour with increased receptor numbers and reduced hormone-binding proteins.
• Placental hormone increase linked to fetal adrenal gland maturation and uterine stretching.

Key Hormones and Mediators

Corticotrophin-Releasing Hormone (CRH)

• Important for labour onset; produced in various body parts (placenta, hypothalamus).
• Dual role: promotes uterine quiescence during pregnancy and contraction during labour.
• Increased levels linked to labour initiation.

Prostaglandins

• Local hormones responsible for uterine contractions and cervical softening.
• PGE2 and PGF2 increase before and during labour.
• Synthesised by decidua, cervix, placenta, and fetal membranes.

Progesterone

• Suppresses uterine excitement by promoting calcium uptake.
• Decreased local activity allows oestrogen to dominate and enable contractions.

Nitric Oxide

• Maintains myometrial quiescence; interacts with progesterone.
• Levels decrease near term in myometrium but increase in cervix.

First Stage of Labour

• Myometrial contractions lead to effacement and dilation of the cervix.
• Divided into latent (slow dilation) and active phases (strong contractions).
• Effective contractions require softened cervix, coordinated contractions, and descending uterine activity gradient.

Uterine Contractions in First Stage

• Initiated by pacemaker cells, facilitated by gap junctions for synchronous contractions.
• Stronger in the fundus, lesser in the lower segment due to muscle distribution.
• Retraction allows continuous shortening of lower uterine segment, aiding effacement.