Physiology of Labor

Initiation of Labor - Hormonal Harmony Kicks

Complex maternal-fetal interplay initiates labor:

• Hormonal Cascade:
  • ↑ Estrogen/Progesterone (E/P) ratio: Key. Estrogen ↑ oxytocin receptors, PGs, gap junctions (Connexin 43). Progesterone influence ↓ (functional withdrawal).
  • Fetal HPA Axis & CRH: Fetal HPA axis (cortisol); placental CRH ↑, boosting fetal DHEAS then estrogen.
• Prostaglandins (PGs):
  • $PGE_2$: Essential for cervical ripening (softening, effacement).
  • $PGF_{2\alpha}$: Potent myometrial contractor, stimulates contractions.

  ⭐ PGs: $PGE_2$ for cervical ripening, $PGF_{2\alpha}$ for myometrial contractility.

• Oxytocin: From posterior pituitary; uterine receptors ↑ significantly at term, enhancing contractility.
• Mechanical Triggers:
  • Uterine Stretch: Myometrial distension (fetus/amniotic fluid) activates pathways.
  • Ferguson Reflex: Cervical stretch (presenting part) → neurohormonal reflex → ↑ oxytocin. 📌 "Fetus Pushes, Oxytocin Gushes."
• Gap Junctions: Increased Connexin 43 allows synchronized, effective uterine contractions.

Stages of Labor - The Labor Show

• First Stage (Cervical Dilatation 0-10 cm): Begins with onset of true labor pains, ends with full cervical dilatation.
  • Latent Phase (0 up to 6 cm): Cervical effacement and early dilatation. Duration variable.
  • Active Phase (6-10 cm): Regular, strong uterine contractions. Cervical dilatation rate: Primigravida ≥1.2 cm/hr, Multigravida ≥1.5 cm/hr.
• Second Stage (Fetal Expulsion): From full cervical dilatation (10 cm) until delivery of the fetus.
  • Involves maternal pushing efforts, fetal descent, crowning, and delivery.

  ⭐ Active phase of labor is generally considered from cervical dilatation of ≥6 cm in the presence of regular uterine contractions.

• Third Stage (Placental Expulsion): From delivery of the fetus until delivery of the placenta.
  • Signs of placental separation: 📌 Change in uterine shape (globular), Gush of blood, Cord lengthening.
  • Active management (e.g., uterotonics) recommended to ↓PPH risk.
• Fourth Stage (Postpartum Monitoring): First 1-4 hours after placental delivery.
  • Crucial for monitoring vital signs, uterine tone (to prevent atony), and vaginal bleeding (PPH).

Table: Average Duration of Labor Stages

Mechanism of Labor - Fetal Navigation System

• Fetal Descriptors:
  • Lie: Fetal to maternal long axis (longitudinal, transverse).
  • Presentation: Part entering pelvis (cephalic, breech).
  • Attitude: Fetal posture (e.g., flexion).
  • Position: Relation of presenting part's denominator to maternal pelvis (e.g., LOA, ROA).
• Cardinal Movements: Sequence of fetal adaptations for passage. 📌 Every Darn Fool In Egypt Eats Raw Eggs.
  • Engagement: Biparietal diameter passes pelvic inlet.
  • Descent: Continuous downward movement.
  • Flexion: Chin to chest, smaller diameter presents.
  • Internal Rotation: Occiput rotates anteriorly to pass ischial spines.
  • Extension: Head extends at pubic symphysis, occiput born.
  • External Rotation (Restitution): Head aligns with shoulders.
  • Expulsion: Anterior, then posterior shoulder, then body delivered.
• Pelvic Diameters: Fetus navigates varying maternal pelvic diameters; adequacy is key.
• Asynclitism: Lateral tilting of fetal head to negotiate pelvic planes.

⭐ The most common fetal position at the onset of labor is Left Occipito-Anterior (LOA).

Maternal Adaptation - Mom's Power-Up

• Cardiovascular: Cardiac Output (CO) ↑ by 30-50%. BP ↑ with contractions, normalizes between.

  ⭐ Maternal cardiac output increases by up to 50% during labor.

• Respiratory: O₂ consumption ↑ (20-60%). Hyperventilation causes mild respiratory alkalosis.
• Hematological: Physiological leukocytosis (WBC up to 25,000/mm³). Hypercoagulability (↑factors, ↓fibrinolysis) protects vs PPH.
• Metabolic: Basal Metabolic Rate (BMR) ↑. Tendency towards ketosis (muscular activity, reduced intake).
• Gastrointestinal (GI): Motility ↓, delayed gastric emptying (↑ aspiration risk).
• Renal: Transient proteinuria is common (due to ↑ GFR, physiological stress).

High‑Yield Points - ⚡ Biggest Takeaways

• Labor initiation: Driven by fetal HPA axis, ↑ Estrogen:Progesterone ratio, ↑ Prostaglandins, and ↑ Oxytocin receptors.
• Ferguson reflex: Cervical stretch triggers oxytocin release, reinforcing uterine contractions.
• Three P's: Power (uterine contractions), Passage (maternal pelvis), Passenger (fetus).
• Uterine contractions: Exhibit fundal dominance and are coordinated by gap junctions.
• Cervical changes: Effacement (thinning) and dilatation (opening) are crucial for labor progression.
• Cardinal movements: Essential sequence including engagement, descent, flexion, internal rotation, extension, external rotation, expulsion.