Mechanisms of labour

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

1^st 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 

2^nd 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 1^st 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) 

3^rd 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 Ts: Tone (uterine atony), Tissue (retained placenta/clots), Trauma (to perineum), Thrombin (coagulopathy)
    • Secondary (24 hours–6 weeks) = retained placenta/clots
  • Retained placenta
  • Inversion of uterus

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.

Test your knowledge on pelvic anatomy

What are the anatomical features of a female pelvis (compared to male)?

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