Section 2: Breastfeeding Background

Mammary Gland Anatomy and Physiology


Initiation of lactation is mediated by hormone release. Progesterone, prolactin, oxytocin and cortisol each play a role in milk production and lactation.


  • High levels of progesterone maintain pregnancy and suppress milk production by inhibiting prolactin.
  • When the baby is born and the placenta is delivered, progesterone levels decrease.

Prolactin: (“Milk Production”)

  • Prolactin is produced in the anterior pituitary gland.
  • Prolactin is inhibited during pregnancy by progesterone.
  • Prolactin is stimulated by suckling and milk removal.
  • Prolactin secretion results in milk production.
  • Prolactin levels vary depending on time of day; they are highest after onset of sleep and early morning.
  • Prolactin Inhibitory Factor (PIF) is released from the hypothalamus and it inhibits prolactin release from the pituitary gland.

Oxytocin: (“Let-down”)

  • Oxytocin is produced in the posterior pituitary gland.
  • Infant suckling and nipple stimulation stimulate oxytocin secretion.
  • Oxytocin serves to cause milk ejection or “let-down” (the mother may sense this as a warm sensation or tingling in the breast).
  • Oxytocin also causes uterine contractions early in the postpartum period, shrinking the uterus to its pre-pregnancy state; mothers may experience uterine cramps when let-down occurs.


  • Cortisol is produced in adrenal glands.
  • Cortisol is released in response to stress.
  • High levels of cortisol can delay lactogenesis while low levels and decreased stress improve breastfeeding.

After milk supply is well established, the amount of breast milk produced is primarily a result of the supply and demand process. Nursing often in the first few days, and milk removal serves to up-regulate receptors and position the mother for a better supply longer term. Feedback inhibitor of lactation (FIL) is a protein in breast milk that is important in the supply and demand process of milk production; if the breasts are emptied regularly, there is less FIL present in the breast and more milk will be produced.