28.6 Lactation

Lactation and Breast Anatomy

Lactation is the process by which the mammary glands produce and secrete milk to nourish a newborn. It depends on a coordinated system of hormonal signals and specialized breast structures that develop fully during pregnancy and activate after birth.

Anatomy of Lactating Breasts

The mammary glands are made up of several key structures that work together to produce, store, and deliver milk.

• Alveoli are the functional units of milk production. They're lined with milk-secreting cells called lactocytes, which synthesize and secrete milk. Surrounding each alveolus are myoepithelial cells that contract to squeeze milk out into the ducts. Alveoli cluster together to form lobules (think of them like bunches of grapes).
• Lobules each contain multiple alveoli and drain into a single lactiferous duct.
• Lactiferous ducts carry milk from the lobules toward the nipple. Near the nipple, these ducts widen into lactiferous sinuses, which serve as temporary milk reservoirs.
• The nipple contains the openings of the lactiferous ducts where milk exits. Smooth muscle fibers in the nipple allow it to become erect during stimulation, which helps with infant latching.
• The areola is the pigmented skin surrounding the nipple. It contains Montgomery's glands (also called areolar glands), which secrete a lubricating fluid that protects the nipple and areola from cracking during breastfeeding.

Process of Milk Production

Milk production and delivery involves two key hormones: prolactin (makes the milk) and oxytocin (moves the milk). Here's how the process works:

1. Prolactin from the anterior pituitary gland stimulates lactocytes in the alveoli to synthesize milk. This initiation of milk production is called lactogenesis.
2. When the infant suckles, sensory nerve impulses travel from the nipple to the hypothalamus.
3. The hypothalamus signals the posterior pituitary to release oxytocin into the bloodstream.
4. Oxytocin causes the myoepithelial cells surrounding the alveoli to contract.
5. This contraction forces milk from the alveoli into the ducts and out through the nipple. This is called the milk ejection reflex (or let-down reflex).
6. Ongoing milk production (galactopoiesis) is maintained by regular breast emptying and continued hormonal stimulation. The more frequently the breast is emptied, the more milk is produced. This is a positive feedback loop: suckling drives prolactin release, which drives more milk synthesis.

Breast Milk Composition

Breast milk changes in composition over time to match the infant's developmental needs. It also changes within a single feeding session.

Stages of Breast Milk

• Colostrum is produced in the first few days postpartum. It's thick, yellowish, and sometimes called "liquid gold." Colostrum is rich in immunoglobulins (especially secretory IgA), which coat the infant's GI tract and provide passive immunity. Compared to mature milk, colostrum is higher in protein but lower in fat and lactose.
• Transitional milk appears from roughly days 3 to 14 postpartum. During this period, milk volume increases and lactose content rises to meet the infant's growing energy demands. Protein and immunoglobulin concentrations gradually decrease as the infant's own immune system begins to develop.
• Mature milk is produced from about day 14 onward and has a relatively stable composition. It contains a balanced mix of carbohydrates, proteins, fats, vitamins, and minerals. Lactose is the primary carbohydrate, providing energy and promoting calcium absorption. Lactoferrin, an iron-binding protein with antimicrobial properties, is also present.

Foremilk vs. Hindmilk

Within a single feeding, the composition of milk shifts:

• Foremilk comes at the beginning of a feeding and is lower in fat (more watery).
• Hindmilk comes toward the end of a feeding and is significantly higher in fat. This higher fat content promotes satiety and supports weight gain.

This is why allowing the infant to fully empty one breast before switching to the other helps ensure they receive adequate hindmilk.

Common Lactation Challenges

• Breast engorgement occurs when breasts overfill with milk, causing swelling, discomfort, and sometimes difficulty with milk flow. Frequent feeding or expressing milk helps relieve engorgement and maintain milk production.