Describe the physiological changes that occur in the pregnant person during parturition (labour). (Changes that occur within the foetus are not required).

Master answer

First Stage of Labour

Onset of Regular Uterine Contractions to Full Cervical Dilation (10 cm)

Endocrine and Uterine Changes

• ↑ Oxytocin (posterior pituitary):
  • Stimulates uterine contractions via calcium-mediated pathways
  • Enhanced by Ferguson reflex (positive feedback from cervical stretch)
• ↑ Prostaglandins (PGE2, PGF2α):
  • Promote cervical ripening by enzymatic degradation of collagen and increased water content
  • Enhance myometrial contractility
• ↑ Estrogen:Progesterone ratio:
  • Increases oxytocin receptor expression in myometrium
  • Reduces uterine quiescence
• Relaxin softens the cervix and pelvic connective tissues

Uterine and Cervical Physiology

• Myometrial contractions begin in the fundus and progress caudally
  • Frequency: ~2–3 per 10 min early, increasing to 4–5 per 10 min by late first stage
  • Duration increases from 30–60 to 60–90 seconds
• Cervical effacement and dilation proceed:
  • Effacement: cervix thins from ~3 cm to a thin membrane
  • Dilation: progresses from 0 to 10 cm

Cardiovascular Changes

• Cardiac output increases by ~30–50% above baseline
  • Stroke volume ↑ due to enhanced venous return between contractions
  • HR increases by 15–20 bpm
• Autotransfusion: ~300–500 mL of blood is displaced from uterus into systemic circulation during each contraction
• Systolic BP may rise transiently during contractions

Respiratory Changes

• Oxygen consumption increases by ~40–60%
• Minute ventilation increases by ~50%, driven primarily by increased tidal volume
• Resultant respiratory alkalosis: PaCO₂ decreases to 28–32 mmHg, with a slight ↑ in pH (up to 7.48)

Metabolic Changes

• ↑ Glucose demand for uterine muscle contractions
• Beginning of lactate accumulation due to intermittent anaerobic metabolism during strong contractions
• Mild ↑ in catecholamines and cortisol stimulate gluconeogenesis and energy mobilisation

Gastrointestinal and Neurological

• ↓ Gastric motility and LOS tone → increased risk of nausea/vomiting
• Pain mediated via visceral afferents (T10–L1) from uterine and cervical stretch
• ↑ Endorphins modulate pain and provide some stress-buffering effects

Second Stage of Labour

Full Cervical Dilation to Delivery of the Neonate

Endocrine and Uterine Muscular Changes

• Stronger, more coordinated fundal-dominant uterine contractions
  • Frequency: ~5 contractions per 10 minutes
  • Peak intrauterine pressure: 100–120 mmHg
• Maternal voluntary effort via abdominal muscle contraction and Valsalva manoeuvre aids fetal descent

Neurohormonal and Pain Modulation

• Somatic pain (S2–S4) due to distension of lower birth canal, perineum, and pelvic floor
• Further ↑ in β-endorphins from hypothalamus and pituitary provide endogenous analgesia
• Sympathetic activation → ↑ catecholamines (adrenaline, noradrenaline)
  • Excessive stress can inhibit contractions and uteroplacental blood flow

Cardiovascular Changes

• Cardiac output peaks at 80–100% above pre-labour baseline
• Increased systemic blood pressure and heart rate during maternal pushing
• Risk of aortocaval compression in supine position → reduced venous return and hypotension; left lateral positioning recommended

Respiratory Changes

• ↑ Oxygen consumption approximates levels seen in moderate-to-heavy exercise
  • Reaches 2.0–2.5 mL/kg/min
• Hyperventilation continues; persistent hypocapnia may lead to lightheadedness, tingling

Metabolic and GI Changes

• High lactate production in uterine muscle → risk of maternal acidosis in prolonged labour
• Glucose depletion in long second stages contributes to maternal fatigue
• Vomiting more likely due to raised intra-abdominal pressure and gastric stasis

Third Stage of Labour

Delivery of the Placenta

Endocrine and Uterine Haemostasis

• Oxytocin surges post-delivery:
  • Promotes strong myometrial contraction
  • Facilitates placental separation
• Prostaglandins also contribute to sustained myometrial tone
• Contraction of spiral arteries and “physiological ligature effect” from muscle fibres reduces haemorrhage
• Active management (e.g. oxytocin IM) used prophylactically to prevent postpartum haemorrhage

Cardiovascular Adjustments

• Blood loss ~300–500 mL in vaginal delivery
  • May exceed 500 mL in postpartum haemorrhage
• Cardiovascular system compensates via vasoconstriction, ↑ HR, and continued autotransfusion from contracting uterus

Respiratory and Metabolic

• Respiratory demand begins to decline as physical effort ceases
• Continued elevation in metabolic rate for tissue repair and recovery
• Restoration of acid–base balance after lactate clearance

Neurological and GI

• Emotional relief and oxytocin release facilitate bonding and lactation initiation
• Endorphin levels remain elevated immediately postpartum
• Risk of vomiting persists due to residual gastric stasis and ongoing hormonal effects