Hormonal Regulation of Lactation: Prolactin, Oxytocin, Estrogen, Progesterone, and the Suckling Reflex

Lactogenesis is a tightly regulated neuroendocrine process. Four primary hormones orchestrate mammary development, milk synthesis, and ejection.

1. Prolactin (PRL) – Milk Synthesis

• Source: Anterior pituitary lactotrophs.
• Action: Induces alveolar differentiation; upregulates casein, lactalbumin, lactose synthetase.
• Pattern: Rises 10–20 fold in pregnancy; suppressed by placental estrogen/progesterone. Postpartum progesterone withdrawal unleashes lactogenesis II (copious milk).
• Regulation: Suckling → afferent spinal → hypothalamus → disinhibition of PRL release (dopamine tonic inhibition lifted). Pulsatile surges with each feed.

2. Oxytocin (OT) – Milk Ejection (Let-Down)

• Source: Posterior pituitary (synthesized in paraventricular/supraoptic nuclei).
• Action: Contracts myoepithelial cells → alveolar compression → milk expulsion into ducts.
• Reflex Arc: Nipple stimulation → spinal cord → hypothalamus → OT release (neuroendocrine reflex). Conditioned to cues (baby cry, smell).
• Inhibitors: Stress (catecholamines), pain, alcohol, nicotine, separation.

3. Estrogen & Progesterone – Mammary Preparation (Pregnancy)

• Estrogen: Ductal elongation, branching, stromal proliferation.
• Progesterone: Alveolar budding, lobuloalveolar development; suppresses lactogenesis via PRL receptor blockade.
• Withdrawal at delivery (placental expulsion) = permissive signal for lactogenesis II.

4. Autocrine Control (Local)

Feedback Inhibitor of Lactation (FIL) in milk accumulates with stasis → downregulates synthesis locally. Explains supply-demand matching.

Clinical Pearls

• Early, frequent suckling (8–12×/24h) establishes robust PRL receptor sites.
• Skin-to-skin contact enhances OT pulsatility.
• Retained placental fragments → sustained progesterone → failed lactogenesis II.
• Dopamine agonists (cabergoline) suppress PRL for suppression; domperidone (off-label) enhances PRL for induction.