Understanding the Anatomy of the Lactating Breast

This article is part of the Nella Vosk Breastfeeding Nutrition & Milk Supply Guide — the comprehensive resource for Australian mothers navigating lactation nutrition. This article addresses one foundational question: how does the lactating breast actually work? Understanding the anatomy and physiology behind milk production helps you make sense of why certain nutrition habits, feeding patterns, and behaviours directly affect your supply.

When breastfeeding is going well, it feels almost miraculous: your body produces exactly the right nutrition for your baby, adapts as they grow, and responds to demand in real time. When it’s challenging, it can feel frustratingly opaque — why isn’t there more milk? What is let-down, exactly? Why does stress interfere? Why does more frequent feeding help?

The answers live in the anatomy and physiology of the lactating breast. Once you understand how the structures and hormones work, breastfeeding challenges become significantly more explainable — and often, more solvable.

At Nella Vosk, supporting the biological foundations of lactation sits at the heart of our Milk & Feeding Support pathway — because when you understand what your body is doing, you can support it far more effectively.

The lactating breast is a remarkably specialised glandular organ. Outside pregnancy, breast tissue is composed primarily of fat, connective tissue, and a relatively small proportion of glandular structures. During pregnancy and lactation, the glandular component expands dramatically to meet the demands of milk production.

Alveoli: where milk is made

The alveoli are tiny, grape-like sacs lined with milk-secreting cells called lactocytes. This is where breast milk is synthesised — where water, proteins, fats, sugars, antibodies, and hundreds of other bioactive compounds are assembled from the nutrients circulating in the mother’s bloodstream. Clusters of alveoli form structures called lobules, and groups of lobules form lobes. Each breast contains 12–20 lobes.

Surrounding the alveoli are myoepithelial cells — small muscle cells whose contraction is triggered by the hormone oxytocin. When these cells contract, they squeeze milk out of the alveoli and into the duct system. This is the physiological mechanism of let-down.

Lactiferous ducts: the milk transport network

Each lobe connects to its own lactiferous duct (milk duct), which carries milk from the alveoli toward the nipple. Research led by Dr Donna Geddes at the University of Western Australia — using ultrasound imaging of the lactating breast — significantly updated our understanding of duct anatomy in the early 2000s. The new model revealed that ducts are considerably more superficial (closer to the skin surface) than older anatomical diagrams suggested, and that approximately 65% of glandular tissue lies within 30mm of the nipple. This has practical implications for latch, hand positioning during expression, and breast shield fit when pumping.

Nipple and areola

Milk exits through multiple small pores in the nipple rather than through a single central channel. The areola — the pigmented circle of skin surrounding the nipple — contains small raised bumps called Montgomery glands (or Montgomery’s tubercles). These secrete a lubricating, antimicrobial oil that protects the nipple and areola from cracking and infection, and also releases subtle scent signals that help guide a newborn to the breast. This is one of the reasons washing the nipple area with harsh soap is not recommended during breastfeeding — it removes a protective and functional secretion.

Blood supply

Milk is made from the mother’s blood. The breast has a rich network of blood vessels that deliver water, glucose, amino acids, fatty acids, vitamins, minerals, and immune factors to the lactocytes. This is why maternal nutrition directly affects the nutritional composition of breast milk — the lactocytes are drawing on whatever is circulating in the bloodstream.

Two hormones are central to lactation: prolactin and oxytocin. Understanding what each does — and what interferes with them — explains much of what new mothers experience.

Prolactin: the milk-making hormone

Prolactin is produced by the anterior pituitary gland and is responsible for stimulating the lactocytes to synthesise milk. Prolactin levels rise significantly during pregnancy, but are suppressed by high oestrogen and progesterone. Following the delivery of the placenta, oestrogen and progesterone drop sharply, and prolactin takes over as the dominant driver of milk production. This is the hormonal trigger for mature milk “coming in” — usually between days 3 and 5 after birth.

Prolactin is released in response to nipple stimulation and milk removal. The more frequently and thoroughly milk is removed from the breast — either by feeding or by pumping — the higher prolactin levels remain, and the more milk is produced. This is the physiological basis of the supply-and-demand principle that underpins almost every evidence-based approach to building and maintaining milk supply.

Prolactin also suppresses ovulation by reducing luteinising hormone (LH) and follicle-stimulating hormone (FSH) — which is the mechanism behind lactational amenorrhoea, the temporary absence of menstruation during breastfeeding.

Oxytocin: the let-down hormone

Oxytocin is produced by the posterior pituitary gland and triggers the contraction of myoepithelial cells around the alveoli, ejecting milk into the ducts. This is the let-down reflex — sometimes felt as a tingling, tightening, or a sense of fullness and then release in the breast. Some women feel it clearly; others barely notice it but can observe milk beginning to drip.

Crucially, oxytocin release can be triggered not only by physical stimulation but also by sensory cues: seeing the baby, hearing a baby cry (any baby), smelling the baby, or even thinking about feeding. This is why milk often lets down unexpectedly in the early weeks.

Oxytocin is also highly sensitive to inhibition. Stress, anxiety, pain, and embarrassment all activate the sympathetic nervous system in ways that suppress oxytocin release. This is the physiological reason why breastfeeding in a calm, private, comfortable environment is associated with better let-down — and why a mother who is stressed, in pain, or feeling observed and judged may have difficulty letting down, even when her milk supply is adequate.

Feedback inhibitor of lactation (FIL)

A third mechanism regulates milk supply at a local level. When milk accumulates in the alveoli and ducts and is not removed, a protein called the feedback inhibitor of lactation (FIL) signals the lactocytes to reduce production. This is why infrequent feeding, skipped feeds, or incomplete breast drainage can reduce supply over time — FIL builds up and down-regulates production. Conversely, frequent and complete milk removal prevents FIL accumulation and maintains production at a high level.

The preparation for lactation begins long before birth. From around 16 weeks of pregnancy, a process called lactogenesis I begins: rising oestrogen and progesterone stimulate the growth and branching of the lactiferous duct system, the development of new lobules and alveoli, and the early production of colostrum. The breast may feel fuller, heavier, or more tender during this period as glandular tissue expands.

By late pregnancy, the alveoli are partially filled with colostrum — the concentrated, immunologically rich first milk. Colostrum production continues regardless of whether the mother ultimately breastfeeds, because it is driven by the hormonal state of pregnancy rather than by suckling.

For a detailed account of what colostrum is, what it does for the newborn, and how the milk stages evolve through the first weeks of breastfeeding, see our article on the stages of breastmilk: colostrum, transitional milk, and mature milk.

Milk is made from the mother’s blood. This is not a figure of speech — the lactocytes in the alveoli actively extract water, nutrients, and immune factors from the maternal bloodstream and synthesise them into breast milk. The nutritional adequacy of what the mother consumes therefore directly affects both what is available for milk production and the composition of the milk produced.

Hydration and milk volume

Breast milk is approximately 87–90% water. Maternal dehydration — even mild dehydration that does not produce obvious symptoms — can reduce milk volume. Lactating mothers have higher fluid requirements than non-lactating adults. Australian dietary guidelines suggest breastfeeding mothers aim for approximately 3 litres of total fluid per day from all sources (food and drinks), though individual needs vary. The most practical approach is to drink to thirst, keep a water bottle visible and accessible at every feeding location, and notice if milk supply drops during periods of reduced fluid intake.

Caloric and protein adequacy

Producing approximately 1.5 litres of milk per day for a single infant requires significant energy and protein. Australian guidelines suggest breastfeeding mothers may need 300–500 additional calories per day above their pre-pregnancy baseline, with protein requirements also elevated. When a mother consistently under-eats — which is common in early postpartum when meals are rushed or forgotten — her body prioritises milk composition over maternal nutritional status. This means the baby continues to receive adequate milk nutrition, but the mother depletes her own reserves more rapidly. Over time this contributes to the postpartum depletion that many mothers experience in the weeks and months after birth.

Fat and milk composition

The fat content of breast milk is influenced by what the mother eats. While total fat content is relatively stable, the fatty acid profile — specifically the ratio of omega-3 to omega-6 fatty acids and the presence of DHA (docosahexaenoic acid) — varies with maternal diet. DHA is critical for infant brain and visual development, and its concentration in milk is higher in mothers who consume oily fish regularly. Continuing a postnatal supplement with DHA throughout breastfeeding is recommended by most Australian healthcare guidelines for this reason.

The oxytocin connection

Oxytocin — the hormone that triggers let-down — is inhibited by stress. Chronic stress, anxiety, pain, and sleep deprivation all elevate cortisol levels in ways that suppress oxytocin release. This is partly a nutritional story: consistent nourishment, adequate blood sugar, and micronutrient adequacy all contribute to a nervous system that is less reactive to stressors. A mother who is running on poor nutrition, chronic sleep debt, and elevated cortisol is physiologically less able to let down easily — which can create a feedback loop where perceived supply problems generate more anxiety, which further inhibits let-down.