The Distillation Process: How Does Breastfeeding Actually WORK?

Support, pictured left. Demand, pictured center. Supply, pictured right. Photo Credit: Ana & Ivan Photography.

Support, pictured left. Demand, pictured center. Supply, pictured right. Photo Credit: Ana & Ivan Photography.

Milk Production: Supply & Demand

Milk-Ejection (Let-Down) Reflex

Transfer of Antibodies

Breast Storage Capacity

Milk Production: Supply & Demand

“In mammals, females dissolve parts of themselves to feed their babies.” – Katie Hinde


Milk production begins during pregnancy. This first milk is called colostrum. Typically colostrum makes way for transitional milk between the second to fifth day postpartum. Over the next week or so, the milk gradually turns into more mature milk, which will start to take on a lighter consistency and bluish-white cast as opposed to the thick yellow-orange of early colostrum. (Read more about the composition of these stages of milk here).

You may be explicitly aware of milk coming in (sudden engorgement), or perhaps not really notice. The timing for when milk comes in differs with each woman and each pregnancy due to various factors, such as type of birth (natural or surgical, medicated or not, number of interventions), level of exhaustion/physical trauma after birth, latching success, frequency of feedings, and whether milk supplementation or dummies (pacifiers) have been used.


1) – Beginning Lactation (Endocrine Control)

  • Occurs in the final three months of a term pregnancy. The hormone prolactin releases at high levels to stimulate milk-making and the multiplying of prolactin receptors in the breasts. This hormone is in the blood during pregnancy and nursing at twenty times higher levels than before pregnancy. (Through the newborn phase, prolactin is ten times higher in a breastfeeding mother than non-breastfeeding mothers whose prolactin levels return to pre-pregnancy levels by two weeks postpartum).
  • Simultaneously during pregnancy, the hormones progesterone and estrogen increase to suppress actual milk availability until the baby is born.

2) – Transition Lactation (Endocrine Control)

  • This begins as the placenta is birthed, spurring dramatically plummeting levels of the milk-suppressing hormones. Prolactin is then unrestrained in its work to produce a foundation of milk irrespective of whether the baby has stimulated supply by attempting to latch.
  • Levels of prolactin are highest in the first ten days post-birth. Starting at about thirty-six hours after birth, milk volume increases ten-fold from about 50 ml/day to 500 ml/day, until about seventy-two hours afterward (‘coming in’).

3) – Established Lactation (Autocrine Control)

  • Oxytocin is the ‘star’ hormone here, as baby’s stimulation of the breasts is now the action that determines milk supply. As long as milk is removed from the breasts, they will keep producing…and producing…and producing. Think of them as Energizer Boobies!
  • A whey protein called Feedback Inhibitor of Lactation (FIL) signals to the body to decrease or increase supply based on its presence in the milk: a fuller breast has more FIL, which slows down production, and an emptier breast has less FIL, which hastens production.
  • This is why it’s crucial for a mother to feed frequently to preserve her supply–especially during the first few weeks. Not only does nursing often (including comfort nursing, not just feeding) meet the needs of the baby, this is when and how your body is told how many milk receptor sites it needs to sustain this child in the future. A limited number of milk receptors laid down during this unique window of opportunity can result in breastfeeding challenges later.



Milk Ejection (Let-Down) Reflex

The let-down reflex can occur several times in a single feeding, though it may only occur once (and perhaps without notice). Some women don’t recognize any sensation accompanying let-down at all, while others experience sudden fullness at the front of the breast, tingling, or milk ‘tugging’ downward through the breast. As a baby gets older, it’s normal to no longer feel the milk letting down and is not itself a sign of low milk supply.

In the first days after birth, let-down also causes uterine contractions during a nursing session (called afterpains), which can hurt — but remember that the contractions are actually helping your uterus shrink back into shape quickly.

Oxytocin, the hormone associated with Milk-Ejection Reflex (MER or let-down), does not require baby’s sucking action to be released. It’s the ‘love hormone’ that floods the body when a mother holds her baby, thinks about her baby, hears her baby, smells or sees her baby. These are also the things that cause the MER to get things flowing in mama’s milk moats.


Dr. William and Martha Sears say that when a mother believes she ‘doesn’t have enough milk,’ usually it’s not actually a supply problem — it’s a MER problem, as in her milk-release reflex just isn’t functioning properly.

Why would this happen? Problems with the MER reflex can signal an interference with proper concentrations of oxytocin in the mother. Kelly Bonyata lists the possible culprits of a weak let-down: anxiety, pain, embarrassment, stress, cold, caffeine, smoking, alcohol, some medications, nerve damage from prior breast surgery, and adrenaline produced in the body in response to extreme crisis situations (learn more about working with a slow let-down).

This is why nursing in public or around less-than-supportive people (anxiety and embarrassment) can literally put a temporary lockdown on mother’s ability to release milk right when her baby needs it.

Are you a ‘nervous type’ or especially affected by anxiety? Though stress can inhibit MER, remember that the milk-ejection reflex is supposed to be a natural relaxant.

MER helps drag the fattier hindmilk past the ducts, so a consistently weak MER won’t necessarily lead to lower supply but can result in the baby receiving too much foremilk. This won’t pose a problem infrequently, but a consistent lack of hindmilk can make it seem like the baby isn’t thriving. Read more about balancing foremilk/hindmilk here.

Transfer of Antibodies

The breast makes customized antibodies as the baby communicates to the nipple (via mouth contact) which pathogens and viruses he was exposed to. The breasts then provides the specific protective elements needed in the milk by the very next feeding.

“In developing countries the most dangerous period of a child’s life begins with weaning when the protection of the breast milk vanishes and often heavily contaminated food is introduced.” – Hanson and Söderström study

Here’s a quick snapshot into the multitude of research that discusses the mucosal immunology of breast milk: “Immunity in human milk” search results (PubMed, U.S. National Library of Medicine National Institutes of Health). Read more about medicine in breast milk.


Antibodies in breast milk target the infectious pathogens in the mother’s habitat, which are the ones most likely to meet the baby after birth. These antibodies/bioactive factors continue to protect infants and children for the entire duration of breastfeeding, until the child’s intestinal immune system begins producing its own.

[B]reast-feeding represents an ingenious immunological integration of mother and child. The mucosae are favored as portals of entry by most infectious agents, and the neonatal period is particularly critical in this respect.” – Brandtzaeg

“Following breastfeeding termination there may be prolonged protection against infections due to influences on the infant immune system mediated via human milk.” – Oddy review

“Disruption of the [microbiota factor] triad can increase the risk of infectious, allergic and inflammatory diseases of the intestine.” – Kaetzel


Newborns are deficient in secretory IgA, an antibody that protects their mucosal membranes. Mother’s milk provides 0.25-0.5 grams of secretory IgA antibodies designed to further develop the newborn immune system. Secretory IgA molecules bind to harmful microorganisms and prevent them from reaching baby’s body tissues. These molecules especially have it out for the most likely immunity saboteurs, those that lurk in the environment where the baby and mother spend most of their time.

“Each antibody [the mother] makes is specific to that agent; that is, it binds to a single protein, or antigen, on the agent and will not waste time attacking irrelevant substances… [also,] the antibodies delivered to the infant ignore useful bacteria normally found in the gut. This flora serves to crowd out the growth of harmful organisms, thus providing another measure of resistance.” – “How Breast Milk Protects Newborns” by Dr. Jack Newman


“Differing from humans, IgG from breast milk in many animal species (rodents, bovines, cats, ferrets, etc.) are transported across the intestinal epithelium into the neonatal circulation.” – P. Van De Perre study

“In humans, in whom gut closure occurs precociously, breast milk antibodies do not enter neonatal/infant circulation. A large part of immunoglobulins excreted in milk are IgA that protect mainly against enteric infections.”- P. Van De Perre study


A newborn’s entire intestinal tract is sterile at birth. The first feed determines which bacteria get through the baby belly door first. Certain bacteria like bifidobacteria are the good guys that you want to claim the territory and set up shop first. High amounts of troublesome types — such as enterobacteriaceae, enterococci, bacteroides, staphylococci, lactobacilli and clostridia — don’t naturally belong in a newborn’s open gut.

Exclusively breast-fed infants show very low or virtually absent populations of the latter types, while beneficial bifidobacteria accounts for more than 90% of all bacteria in breast-fed baby’s intestines. Gut flora changes to look more like the environment of a bottle/formula-fed baby’s gut when solids or other liquids are introduced and bacteria enterics, bacteroides, enterococci lactobacilli and clostridia start hanging out in much larger crowds.

“Apparently, human milk contains a growth factor that enriches for growth of bifidobacteria, and these bacteria play an important role in preventing colonization of the infant intestinal tract by non indigenous or pathogenic species.” – “The Normal Bacterial Flora of Humans”

By the age of weaning, mice that received maternal SIgA in breast milk had a significantly different gut microbiota from mice that did not receive SIgA, and these differences were magnified when the mice reached adulthood. Early exposure to SIgA in breast milk resulted in a pattern of intestinal epithelial cell gene expression in adult mice that differed from that of mice that were not exposed to passive SIgA, including genes associated with intestinal inflammatory diseases in humans.” –  Rogier, Frantz, et al


In the rotavirus model, antirotaviral IgA was found in stools of breast-fed but not bottle-fed babies. It was further found that rotavirus specific IgA is selectively, locally produced in breast tissue.

“Human milk also contains anti-idiotypic antibodies capable of enhancing infant antibody response. Maternal milk antibodies coat infant mucosal surfaces and some have a clear protective role. This has been studied extensively in infectious disease models such as rotavirus, E. coli, poliovirus, and retroviruses.” – P. Van De Perre study

Breast Storage Capacity

This refers to how much milk a breast can hold between feedings. Breast storage capacity differs between women, and even between an individual’s own breasts.

High or low capacity is not related to breast size (amount of fatty tissue is irrelevant to milk-making), so large breasts won’t necessarily hold more milk at a time than small breasts and vice versa.

Is it better to have a larger storage capacity? Not necessarily.

KellyMom explains it like this:

“Think of storage capacity as a cup – you can easily drink a large amount of water throughout the day using any size of cup – small, medium or large – but if you use a smaller cup it will be refilled more often.”


Breasts are never truly ’empty,’ though you’ll often hear about “when breasts are empty, do this or that…”

They actually produce milk constantly in a continuum, which “ranges from its full capacity (varies from woman to woman and depends on the amount of milk-producing glands in the breast and functioning, intact milk ducts, among other factors) to an amount that is so small as to not be appreciable from outside the breast,” according to The International Breastfeeding Symbol, which further explains:

“[P]icture a sink filled with water and the tap running slowly. This is like the full breast; the milk being produced is represented by the tap, very slow when the breast is full. If you pull the plug, the water will run down the drain, but as you do so, picture the water coming out of the tap faster. This is analogous to feeding the baby; the water running down the drain is the milk being fed to the baby, while the tap increasing its flow is a lot like the milk being produced in the breast as it empties — it increases also.”