Lactation physiology

Hormonal Control - Prepping the Factory

• Pregnancy: High estrogen & progesterone drive development.
  • Estrogen: Stimulates mammary duct proliferation.
  • Progesterone: Promotes alveolar-lobular growth.
  • Prolactin: Rises, but its lactogenic effect is blocked by high estrogen/progesterone at the receptor level.
• Postpartum: Abrupt ↓ in estrogen & progesterone uninhibits prolactin.
  • Prolactin: Initiates & sustains milk synthesis (lactogenesis).
  • Oxytocin: Mediates milk ejection ("let-down") via myoepithelial cell contraction.

📌 Prolactin → Produces milk; Oxytocin → Outflow of milk.

⭐ Prolactin inhibits GnRH, leading to suppression of ovulation (lactational amenorrhea) and decreased fertility in breastfeeding mothers.

Lactogenesis & Ejection - The Milk Let-Down

• Lactogenesis (Production): Triggered by ↓ progesterone post-delivery, allowing prolactin (anterior pituitary) to stimulate milk synthesis in alveolar cells.
• Ejection (Let-down): Suckling sends neural signals to the hypothalamus, triggering oxytocin release from the posterior pituitary. Oxytocin induces myoepithelial cell contraction, expelling milk.
• 📌 Prolactin → Production; Oxytocin → Outflow.

⭐ Prolactin suppresses GnRH release, leading to lactational amenorrhea and anovulation.

Milk Composition - What's on the Menu?

• Colostrum (First 2-3 days): "Liquid Gold"
  • Thick, yellowish fluid
  • ↑ Protein, ↓ Fat/Lactose vs. mature milk
  • Rich in immunoglobulins (secretory IgA), lactoferrin, growth factors, and leukocytes.
• Mature Milk (From day 3-5)
  • Fore-milk: Watery, low-fat; quenches thirst.
  • Hind-milk: Creamy, high-fat; provides energy.
  • Key Components:
    • Carbs: Lactose (main energy source)
    • Lipids: Triglycerides, DHA/ARA (for brain development)
    • Protein: Whey > Casein (~60:40). 📌 Whey is Way more in early milk.
    • Immune: IgA, lysozyme, lactoferrin.

⭐ Exclusively breastfed infants require Vitamin D supplementation (400 IU/day) starting shortly after birth to prevent rickets. Vitamin K is given at birth to prevent hemorrhagic disease.

Clinical Pearls - When Lactation Goes Wrong

• Galactorrhea: Inappropriate milky discharge.
  • Causes: Prolactinoma (most common), dopamine antagonists (e.g., antipsychotics), hypothyroidism (↑ TRH stimulates prolactin).
  • Workup: Check prolactin, TSH, β-hCG levels; consider pituitary MRI.
• Mastitis: Unilateral breast pain, erythema, fever during breastfeeding.
  • Pathogen: S. aureus.
  • Tx: Continue nursing/pumping + antibiotics (e.g., dicloxacillin, nafcillin).
• Breast Abscess: Fluctuant, tender mass (a complication of mastitis). Requires drainage.
• Agalactia/Hypogalactia: Insufficient milk production.
  • Causes: Sheehan syndrome, retained placental fragments.

⭐ High-Yield: Prolactinomas are a key cause of galactorrhea. The resulting hyperprolactinemia suppresses GnRH, leading to amenorrhea, osteoporosis, and infertility.

High‑Yield Points - ⚡ Biggest Takeaways

• Prolactin from the anterior pituitary governs milk production (lactogenesis).
• Oxytocin from the posterior pituitary controls milk letdown (ejection).
• Suckling provides the primary stimulus, inhibiting dopamine to ↑ prolactin and stimulating oxytocin release.
• High progesterone and estrogen during pregnancy inhibit prolactin's effect on the breast.
• The postpartum drop in progesterone allows lactation to begin.
• Prolactin inhibits GnRH, leading to lactational amenorrhea.