Lactation physiology

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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.

Lactation: Prolactin and Oxytocin Positive Feedback Loop

Lactogenesis & Ejection - The Milk Let-Down

Neurohormonal control of lactation and 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.

Nutrient composition of colostrum, mature milk, and formulas

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.

Types of Breast Abscesses

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.

Practice Questions: Lactation physiology

Test your understanding with these related questions

A 26-year-old woman comes to the emergency department because of a 3-day history of nausea and vomiting. Her last menstrual period was 9 weeks ago. A urine pregnancy test is positive. Ultrasonography shows an intrauterine pregnancy consistent in size with a 7-week gestation. The hormone that was measured in this patient's urine to detect the pregnancy is also directly responsible for which of the following processes?

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Flashcards: Lactation physiology

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_____ stimulates breast development in a supportive role with estrogen

TAP TO REVEAL ANSWER

_____ stimulates breast development in a supportive role with estrogen

Prolactin

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