Parturition

Initiation of Labor - Go Time Signals

• Mechanical Factors: Uterine overdistension → ↑contractility.
• Fetal Endocrine Signals:
  • Fetal Hypothalamic-Pituitary-Adrenal (HPA) axis maturation: ↑Fetal cortisol → ↑placental Corticotropin-Releasing Hormone (CRH).
  • Placental CRH: Positive feedback, ↑Dehydroepiandrosterone Sulfate (DHEAS) → ↑estrogens.
• Hormonal Shift: ↑Estrogen / ↓Progesterone Ratio
  • Estrogen: ↑Gap junctions (connexin-43), ↑oxytocin receptors, ↑prostaglandin (PG) synthesis.
  • Progesterone: Functional withdrawal (altered receptor activity, not just ↓levels).
• Prostaglandins (PGE₂, PGF₂α):
  • Key for cervical ripening (PGE₂) & myometrial contractions (PGF₂α, PGE₂).
  • Synthesized in amnion, chorion, decidua.
• Oxytocin:
  • ↑Myometrial contractility (via IP₃/DAG pathway).
  • Ferguson Reflex: Cervical stretch → ↑oxytocin release (positive feedback).

⭐ Placental CRH acts as a "placental clock"; its levels rise exponentially in the third trimester, influencing the timing of parturition and fetal lung maturation.

Stages of Labor - The Parturition Path

• Stage 1: Cervical Dilation (True labor pains → Full dilation 10 cm)
  • Latent Phase: 0-4 cm (Primigravida: ~8h, Multigravida: ~5h)
  • Active Phase: 4-10 cm (Dilation: P: ~1.2 cm/hr, M: ~1.5 cm/hr)
• Stage 2: Fetal Expulsion (Full dilation → Fetal delivery)
  • Duration: P: ~2h (epidural ~3h); M: ~1h (epidural ~2h)

  ⭐ Crowning: Biparietal diameter distends vulva; no recession.

• Stage 3: Placental Expulsion (Fetal delivery → Placental delivery)
  • Duration: 5-15 min (>30 min abnormal)
  • Signs: Blood gush, cord lengthens, uterus firms & rises.
• Stage 4: Observation (1-4h postpartum)
  • Monitor: Uterine tone, bleeding (PPH).

Mechanism of Labor - Baby's Exit Route

Sequential fetal movements for birth canal passage:

• Engagement: Fetal head's biparietal diameter passes pelvic inlet.
• Descent: Continuous downward movement throughout labor.
• Flexion: Fetal chin tucks to chest, presenting smaller diameter (suboccipitobregmatic).
• Internal Rotation: Occiput rotates anteriorly towards pubic symphysis.
• Extension: Head extends as occiput passes under pubic symphysis; crowning.
• Restitution: Head externally rotates to align with shoulders.
• External Rotation: Shoulders rotate into antero-posterior pelvic diameter.
• Expulsion: Anterior shoulder, then posterior shoulder, then body deliver.

📌 Mnemonic: Every Darn Fool In Rotterdam Eats Rotten Eggs (for the cardinal movements)

⭐ The Left Occipito-Anterior (LOA) is the most common and favorable fetal position.

Maternal Physiology & Pain Relief - Mom's Labor Load

• Maternal Physiology:
  • CV: ↑CO (~50% 2nd stage), ↑BP. Risk: Supine hypotension.
  • Resp: Hyperventilation → resp. alkalosis. ↑O₂ consumption.
  • Haem: Physiologic leukocytosis (WBC up to 25k-30k/mm³).
  • GI: ↓Gastric emptying (aspiration risk).
• Labor Pain Pathways:
  • 1st Stage: T10-L1 (visceral - uterine/cervical).
  • 2nd Stage: S2-S4 (somatic - perineal).
• Pain Relief in Labor:
  • Non-Pharmacological: Lamaze, TENS, hydrotherapy, breathing.
  • Pharmacological (Systemic):
    • Inhalational: Entonox (50% N₂O + 50% O₂).
    • Opioids: Pethidine, Fentanyl (⚠️ neonatal resp. depression).
  • Regional Analgesia (Most Effective):
    • Epidural: Bupivacaine + Fentanyl. S/E: hypotension, ↑2nd stage, headache.
    • Spinal: C-section, rapid analgesia.
    • Pudendal block: Perineal pain, episiotomy.

⭐ Epidural analgesia, while highly effective, is associated with an increased risk of instrumental delivery and a longer second stage of labor.

High‑Yield Points - ⚡ Biggest Takeaways

• Ferguson reflex: cervical stretch → oxytocin release (posterior pituitary).
• Oxytocin & Prostaglandins (PGF2α, PGE2): drive uterine contractions; prostaglandins also ensure cervical ripening.
• ↑ Estrogen/Progesterone ratio: sensitizes uterus to oxytocin.
• Stages of Labor: 1st (cervical dilation), 2nd (fetal expulsion), 3rd (placental delivery).
• Relaxin: aids cervical softening & pelvic ligament relaxation.
• Lightening: fetal head descent into pelvis, often pre-labor.
• CRH (Corticotropin-Releasing Hormone) from placenta also plays a role in timing parturition by increasing fetal cortisol which promotes surfactant production and lung maturity impacting prostaglandin synthesis indirectly.