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Mechanisms of labour

Labour = expulsion of products of conception after 24 weeks gestation.

1st stage of labour (cervical dilation)

  • Timing: 12-15 hours if primiparous (1cm/2 hours), 7.5 hours if multiparous (1cm/hour). It is usually divided into the latent phase (cervical dilation 0-3cm) and active phase (cervical dilation 3-10cm).
  • Signs:
    • ‘Show’ (passage of blood stained mucus)
    • Regular painful contractions (3-4 in 10 minutes) 
    • Rupture of membranes
  • Mechanism: fetal head descends into pelvis
  • Complications: 3 Ps
    • Passenger: cephalopelvic disproportion, fetal malpresentation (e.g. persistent occipito-posterior position)
    • Passage: fibroids, cervical stenosis, narrow mid-pelvis
    • Power: primary uterine inertia
  • Interventions may include: C-section (e.g. for cephalopelvic disproportion), oxytocin (for contractions)
  • Other points: induction of labour may be required for overdue babies, and it may be initiated by a membrane sweep, prostaglandin gel/pessary or artificial rupture of membranes 

2nd stage of labour (expulsion of foetus)

  • Timing: 45-120 minutes if primiparous, 15-45 minutes if multiparous
  • Signs: first sign is desire to bear down
  • Mechanism: most common starting position is left occiput anterior (see below)
    • Flexed fetus descends: fetal head is very flexed on spine and descends downwards
    • Internal rotation: whole fetus internally rotates (until facing towards maternal back; head at level of ischial spines)
    • Extension of head: head extends around pubic symphysis until delivery
    • Restitution (external rotation): after head delivered, fetus rotates back to its original position (i.e. with shoulders antero-posterior) and comes out sideways
    • Delivery of shoulders: anterior shoulder comes out first, then rest in pelvic axis (i.e. anteriorly)
  • Complications: as 1st stage
  • Intervene when: maternal/fetal distress, incomplete internal rotation causing failure to progress
  • Interventions may include: instrumental delivery/C-section (for fetal distress or failure to progress), oxytocin (for contractions), McRoberts manoeuvre (for shoulder dystocia) 

3rd stage of labour (expulsion of placenta)

  • Timing: Around 5-10 minutes with syntometrine (30 minutes – 1 hour without) – IM syntometrine is usually given when the head is born to reduce time and post-partum haemorrhage risk
  • Signs:
    • Gush of blood (50-100ml)
    • Lengthening of cord
  • Management: controlled cord traction
  • Complications:
    • Post-partum haemorrhage (>500ml blood loss)
      • Primary (within 24 hours) = 4 TsTone (uterine atony), Tissue (retained placenta/clots), Trauma (to perineum), Thrombin (coagulopathy)
      • Secondary (24 hours–6 weeks) = retained placenta/clots
    • Retained placenta
    • Inversion of uterus

Other points

  • Pelvic anatomy
    • Pelvic inlet (brim) formed by: sacral prominence, arcuate and pectineal lines, upper margin of pubic symphysis
    • Pelvic outlet formed by: coccyx tip, sacrotuberous ligament, ischial tuberosities, pubic arch
    • False (greater) pelvis = part of pelvis above pelvic brim
    • True (lesser) pelvis = part of pelvis below pelvic brim
  • Female pelvic features (compared to male)
    • Wider and shallower
    • Round/oval pelvic inlet (male is heart shaped)
    • Larger pelvic outlet
    • Pubic arch >100˚ (male is <90˚)
    • Wider greater sciatic notch
    • Curved sacrum
  • Common fetal orientations
    • Lie: 
      • Longitudinal (baby vertical)
      • Transverse (baby horizontal)
      • Oblique (baby diagonal)
    • Presentation: 
      • Cephalic (head is presenting part)
      • Breech (bottom is presenting part)
    • Position (of occiput relative to pelvic rim):
      • Left/right occiput-anterior (LOA/ROA)
      • Left/right occiput-transverse (LOT/ROT)
      • Left/right occiput-posterior (LOP/ROP) 

NB: left occiput-anterior is most common position.

Occiput positions

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