Mammary Gland Anatomy and Physiology

Physiology:¹⁵

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

Progesterone:

• 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, and lowest at mid-morning.
• Prolactin secretion is a drug target for medications to increase milk supply.
• 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:

• 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 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.