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Milk Production Overview

A condensed excerpt from "The Breastfeeding Mother's Guide to Making More Milk" by Diana West and Lisa Marasco (McGraw-Hill, December, 2008)

How Your Hormones Work in Your Body
The First Week of Milk Production

How Your Body Decides How Much Milk to Make
The Milk Ejection Reflex: How Nature Delivers Stored Milk to Your Baby
The Resource-Efficient Breast
Storage Capacity
Baby Calls the Shots

Human milk production is an amazing process when you examine it closely. While most mammals have fully developed mammary glands prior to pregnancy, the human breast develops in stages and does not reach full operational maturity until pregnancy.  Like a fruit tree in winter with only a few leaves and dormant buds, the non-pregnant, non-lactating female breast has large and small branches called ducts and ductules,  along with a small number of leaves, the milk-making alveoli.  Each of the several main branches (ducts), with its smaller branches (ductules) and leaves (alveoli), is called a lobe and can function somewhat independently of the other large branches. The milk-making alveoli are located along the branches, and during pregnancy, they multiply and fill out the lobes of the breast like a tree leafing out and flowering in the springtime. When baby is born, milk production kicks into high gear and the tree bears its crop of fruit, the milk, while the milk-making cells continue to multiply according to demand during the next several weeks. When baby is older and his need for milk gradually diminishes, the leaves of the tree, the alveoli, begin to wither and die. This is the autumn season of the breast when there is still milk-making activity, but at a lower level.(1)  At weaning, the breast gradually returns to the resting winter state. This process repeats itself with every pregnancy.

The driving forces underlying this cycle are the hormones related to puberty, pregnancy, and lactation. From puberty on, the waxing and waning (think of moon phases) of estrogen and progesterone during the menstrual cycle slowly develops the ducts and alveoli; this subtle stimulation of the glands continues until about age 30. Pregnancy stimulates a large rise in both of these hormones, as well as the production of prolactin, human placental lactogen, human chorionic gonadotropin (HCG), and growth hormone, which all help to stimulate alveolar growth. Research has shown that the changes in breast size (volume) that women experience during pregnancy are most closely related to the concentration of human placental lactogen, which is produced only by the placenta and therefore only during pregnancy.(2)  At the same time, the glandular tissue of the breast also becomes sensitive to insulin, the hormone that helps bring fuel (glucose) to the cells, while also playing a role in the making, or synthesizing, of milk. Full maturation of the milk-making tissue requires the hormones insulin, cortisol, thyroxine, prolactin, and growth hormone.
How Your Hormones Work in Your Body

One aspect of hormones that is often largely neglected is the understanding of how they work. Hormones have no influence in the body without receptors, to which hormones bind in order to affect the cells where the receptors are located. Hormones can only bind to their own (or sometimes very similar) receptors, like keys that fit only into locks that are configured for them. There must also be a good match between the number of keys to the number of locks—a lot of keys and few locks are not effective, and a few keys but a lot of locks are not effective. The number of available receptors is dynamic—changeable—and is influenced by different factors for different hormones; for example, prolactin helps to stimulate the creation of estrogen receptors. During pregnancy and lactation, the number and binding ability of hormone receptors important for breast development and milk production change according to a combination of their genetically programmed timetables and these other influences.

The First Week of Milk Production

There are many hormones that play a role in the start-up of milk production, a process called lactogenesis. Like a good cake recipe, some ingredients (hormones) play minor roles while some are crucial for a good result. Prolactin, the major milk-stimulating hormone, is normally present in small amounts in our bodies. During pregnancy, prolactin gradually reaches very high levels, multiple times its normal level, peaking at the end of pregnancy. The only reason that volumes of milk are not made before birth is that high levels of progesterone interfere with prolactin receptors (“locks”) by not allowing them to multiply.  This prevents the prolactin “keys” from having much effect and allows only small amounts of the first milk, colostrum, to be made. This is known as Lactogenesis I, the time during the second half of pregnancy when only colostrum is produced. Since the placenta makes the majority of the progesterone during pregnancy, once it is delivered after birth, the progesterone level drops quickly, allowing prolactin to start doing its work. Within 30 to 40 hours(3) after birth, the change to full milk production begins.  Around day two to four on average, mothers notice that they are producing more milk that is lighter in color and say that their milk has “come in.”  This stage is known as Lactogenesis II. There are three major hormones considered necessary for Lactogensis II to begin, sometimes referred to as the Lactogenic Complex: prolactin, insulin, and cortisol.(4)

Nature is amazing in how it is constantly tailoring your milk to be just what your baby needs. Colostrum is made during the second half of pregnancy and is the first milk that baby receives after birth. Unlike the later mature milk, it is yellowish in color because it contains a lot of the nutrient beta carotene, which is an anti-oxidant and a building block of Vitamin A.  It is thicker in consistency and small in quantity; babies take between one half to four teaspoons (2-20 mls) per feeding for an average total of 3.38 ounces (100 mls) per day. Colostrum is higher in protein and lower in fat than mature milk, and is densely packed with immunoglobulins, which are antibodies and anti-oxidants that colonize baby’s digestive tract and help inoculate and protect against future bacteria, viruses, fungus, and allergens. Colostrum also has laxative properties that stimulate the bowels to quickly get rid of meconium, baby’s first stool, helping to minimize jaundice in the days after birth.

As full milk production begins when the milk comes in, colostrum gradually lightens in color and becomes more opaque to form transitional milk, which is produced in higher volumes but is still yellowish in color. Transitional milk is halfway between mature milk and colostrum, and gradually increases in fat and volume.  The change from transitional to mature milk lasts from between 7 to 14 days. Mature milk is whiter (though it can vary in color according to mother’s diet) and has the most cream while being 88% water. It contains many immune properties, but they are not as concentrated as in colostrum.  While the recipe for mature milk remains fairly constant, human milk continues to change in smaller ways as baby gets older to meet his ever-changing needs.(5)

You may have heard that mothers produce two kinds of milk, foremilk, the thinner milk the baby gets first, which has a lower fat content, and hindmilk, the high-fat, creamier milk that follows. These terms can make it seem as if the breasts produce two kinds of milk, but this is not the case.  The milk-making cells in the breast actually produce only one type of milk, but the percentage of fat in the milk that is removed varies according to how long the milk has been collecting in the ducts, and how much of the breast is drained at the moment. As milk is made, the fat globules stick to the sides of the alveoli where the milk is stored, while some of the watery portion of the milk moves down the ducts toward the nipple, mixing with any milk left there from the last feeding. The longer the amount of time between feedings, the more diluted that leftover milk becomes. This “watery” milk has a higher lactose content and less fat than the milk stored in the alveoli higher up in the breast. As baby begins nursing, the first thing he receives is this lower-fat foremilk, which quenches his thirst. Baby’s nursing triggers the mother’s milk-ejection reflex which squeezes milk—and the sticking fatfrom the alveoli into the ducts. The longer he nurses, the higher the fat content of the milk and the more cream he receives, ending up with a nicely balanced meal containing all the fat calories for growth and lactose for energy and brain development that he needs.(6), (7)

Click here for more information about foremilk and hindmilk

How Your Body Decides How Much Milk to Make

So how do the breasts figure out how much milk to make? In the beginning, some mothers seem to initially make enough for two babies and are overflowing with milk, though quite often mothers start off lower and their production gradually increases to meet baby’s needs.(8) Your eventual milk production level is determined by the feeding interactions between you and your baby (or breast pump) and how they influence your hormones.

In the next several weeks after birth, average (or baseline) prolactin levels decrease until they reach a lower plateau. At the same time, receptors for prolactin are multiplying. The development of prolactin receptors in the breast is believed to be related to early frequent suckling stimulation and milk removal: the more often baby breastfeeds in the first days and weeks after birth, the more receptors are developed.  (9), (10), (11)

During the time when prolactin levels are dropping and its receptors are being established, the milk making process transitions from a largely hormone-driven (endocrine) system to one that is controlled more locally in the breast (autocrine).  Under autocrine control, the breast responds to baby’s demand by adjusting the speed of milk production upward or downward according to the amount of milk removed.  The more milk that is stored in the breast, the slower milk is produced; the emptier the breast, the faster the production of milk.  The breast is very sensitive and responsive to the degree of fullness. As the breast fills up, concentrations of a whey protein called the feedback inhibitor of lactation (FIL) rise and trigger a cut-back in the rate of milk production, much like we would slow the water down in a bathtub that is filling up too quickly. Concentrations of FIL are low when the breast is empty, allowing the breast to produce milk faster just as we would turn the water faucet on high when a bathtub is empty and we want to fill it up.

Every time baby suckles at the breast, nerves in the nipple and areola are stimulated, sending messages along nerve pathways to the brain saying that milk is needed. The pituitary responds by releasing a surge of prolactin, causing a temporary increase in prolactin levels to encourage continued re-fueling of the breast. As long as milk is being removed and is not building up in the breast, more milk will be made. More new milk is made when the breast is emptier, and less is made when the breast is fuller.

The goal of the autocrine process is to fine-tune your milk production to meet baby’s actual needs. Like the marketing research department in a factory, your body spends the first few weeks after baby is born determining how big of a factory is needed and how fast it must work to meet baby’s milk needs.(12) In essence, all the experiences of how often baby nurses and how much milk he removes is part of the body’s “market research phase” as it calibrates your milk production.  This period is critical in laying the blueprint for a milk making factory that will meet baby’s needs until he no longer needs it.

The Milk Ejection Reflex: How Nature Delivers Stored Milk to Your Baby

At the same time that baby’s suckling triggers the release of prolactin, it is also triggering the release of oxytocin, the hormone that causes muscle-like cells around the alveoli to contract and squeeze milk into the ducts for delivery to the baby. This process of releasing milk is called milk ejection, which is often less accurately referred to as the “let-down” response. Without this reflex, milk cannot be removed, and when not removed, the breast receives the message to cut back on milk production. For this reason, milk ejection is a critical component of the big picture of milk production.  Fortunately, the process of milk ejection works extremely well the vast majority of time. There are a few situations, such as cigarette smoking or thyroid problems, that can negatively affect milk ejection and lead to lower milk production over time.

Mothers are often surprised to discover that milk ejections are not one-time events during a single feeding session.  The milk ejection reflex can be triggered multiple times during a breastfeeding or pumping session (up to twelve have been recorded,(13) but the average during breastfeeding is 2.5).(14)  The result is several separate spurts of milk flow beginning with an average of one ounce (30 cc) per ejection and decreasing in amount each time as the breast empties.(15) A small number of women may eject a larger quantity of milk at one time; as much as 140 gms (4.67 oz) have been recorded for one ejection. On average, however, the more milk ejections a mother has, the more milk the baby tends to consume during a feeding.(16)  

Babies generally learn to anticipate whatever pattern their mothers’ milk ejections follow.  A hungry baby will continue to suckle off and on when the milk is not flowing strongly, hoping to trigger another ejection. In the early days, however, some babies may become upset if the milk ejection reflex does not happen right away, pulling away in anger or frustration. Over time and with positive experiences, baby learns that the milk really is there (with a normal milk supply), and he learns to trust the breast while his mother learns to trust that her milk will flow, and they both begin to relax into a mutually trusting breastfeeding relationship.  When milk production and flow are low, some babies may remain distrustful until the flow is improved, compounding the difficulty in getting the breastfeeding relationship going.

Oxytocin release is unique in that it is not controlled exclusively by sensory (touch) stimulation, but can also be triggered by thoughts and feelings, as many breastfeeding mothers can attest who heard a baby cry in a shopping center and looked down to see wet spots on their shirts.  This important component of milk production will be explored more deeply in Chapter 10.

The Resource-Efficient Breast

A way of picturing this process of balancing milk production with demand is to compare the breast to a bathtub. When the water level is low, we turn the spigot on full force (A). When the water level is high, we turn it down and let just a little water dribble through (B). When the bathtub is in danger of overflowing, we quickly turn the water off! (C) Milk production is turned back on when some of the milk is allowed to drain out (removed) but will stay turned off permanently if the milk is never drained. This is why a breast that becomes engorged needs to be drained on a regular basis: it is possible to go from too much to very little or no milk in a short time.

When you compare the breast to a bathtub, it’s important to understand that each breast has multiple lobes, so there are actually multiple individual “bathtubs” within each breast. Different lobes can be in different stages according to their degree of fullness, which is why we can never really say that the breast is completely empty.

Even when the baby seems to have drained all available milk from the breast, if he continues to suckle, he will get whatever milk is being made and ejected at the moment. It is as if the bathtub drain is open, but the spigot is on (D). The force of the milk flow is greatly reduced because there is little accumulated milk in the breast.  Thirsty babies tend to prefer a fast flow of milk, so they can become impatient and fussy when the force of the milk flow is reduced, even though the rate of milk production is high.  Mothers often misinterpret this scenario, believing that their breasts are empty and doing nothing. More likely, there is a constant production of milk; it’s just that the milk is being delivered slower than the baby desires.

Storage Capacity

Milk made between feedings is mostly stored in the ductules and alveoli. While the fat droplets generally stick to the sides of the alveoli, some of the watery portion of the milk drips forward and collects in the ductules for the next feeding. The maximum amount of milk that can be stored before the breast says “stop!” is a woman’s storage capacity and can vary widely from one woman to the next, and even one pregnancy to the next (according to baby’s gender and appetite, among other things); one study showed a range of 80 ml to 600 ml.(17)  In addition, storage capacity can change and often increases during the first few months depending upon baby’s demand.(18) The exterior size of the breast is not necessarily a good indicator of storage capacity; some small breasts are densely packed with glandular tissue, while some large breasts are only lightly populated with glands. It is the amount of well-developed glands inside the breast, rather than exterior breast size, that determines storage capacity.

Baby Calls the Shots

It has long been understood that the breast works on a demand and supply process—baby suckles at the breast to demand milk, and the body responds by supplying it to him (via milk ejection) and then replacing what he takes and making even more if baby keeps asking. Calibration, the body’s process of figuring out how much milk to make, is designed to be an infant-driven system. Your body responds to your individual baby with a supply that is tailored to your baby’s specific needs. This is why women may have very different breastfeeding and milk production experiences from one baby to the next; each is a new and unique situation. Similarly, some women may develop a larger milk supply on one side than the other, especially if baby favors one breast over the other.  Interestingly, mothers of baby boys tend to produce more milk than mothers of baby girls, probably due to the fact that baby boys seem to grow a little faster and need a little more milk, thus creating a bigger milk supply in their mothers.(19)

The ability of a woman’s body to respond to baby’s milk-making signals depends not only on baby having access to the breast to send the signals, but also on mother’s body having well-functioning nerves to carry the signals from the breast to the brain for processing. It is truly an amazing, coordinated dance between a mother and her infant.


(1) Kent, J., Mitoulas, L., Cox, D., et al. Breast volume and milk production during extended lactation in women. Exp Phys 1999; 84:435-447.

(2) Cox, D., Kent, J., Casey, T., et al.  Breast growth and the urinary excretion of lactose during human pregnancy and early lactation: endocrine relationships.  Exp Physiol 1999 Mar; 84(2):421-34.

(3) Chapman, D. and Perez-Escamilla, R.  Does delayed perception of the onset of lactation shorten breastfeeding duration?  J Hum Lact 1999 Jun; 15(2):107-11; 107-110.

(4) Lawrence, R. and Lawrence, R.  Breastfeeding: A Guide for the Medical Profession, sixth ed.  Elsevier Mosby: Philadelphia, Pennsylvania. 2005:70.

(5) Lawrence, R. and Lawrence, R.  Breastfeeding: A Guide for the Medical Profession, sixth ed.  Elsevier Mosby: Philadelphia, Pennsylvania. 2005:70.

(6) Lawlor-Smith, C. and Lawlor-Smith, L. Lactose intolerance. Breastfeeding Rev 1998; 6(1): 29-30.

(7) Rings, E. et al. Lactose intolerance and lactase deficiency in children.  Curr Op Ped 1994; 6: 562-67.

(8) Woolridge, M. Problems of establishing lactation. Food & Nutr Bull 1996; 17:316-23.

(9) DeCarvalho, M., Robertson, S., Friedman, A., et al. Effect of frequent breast-feeding on early milk production and infant weight gain. Pediatrics 1983 Sep; 72(3):307-11.

(10) Theil, P., Seirsen, K., Hurley, W., et al. Role of suckling in regulating cell turnover and onset and maintenance of lactation in individual mammary glands of sows. J Anim Sci 2006 Jul; 84(7):1691-8.

(11) Kim, J., Mizoguchi, Y., Yamaguchi, H., et al. Removal of milk by suckling acutely increases the prolactin receptor gene expression in the lactating mouse mammary gland. Molecular and Cellular Endocrinology 1997 Jul; 131(1):31-38.

(12) Woolridge, M. Problems of establishing lactation. Food & Nutr Bull 1996; 17:316-23.

(13) Ramsay, D., Mitoulas, L., Kent, J., et al. Milk flow rates can be sued to identify and investigate milk ejection in women expressing breast milk using an electric pump. Breastfeeding Medicine 2006; 1(1):14-23.

(14) Ramsay, D., Kent, J., Owens, R., et al. Ultrasound imaging of milk ejection in the breast of lactating women. Pediatrics 2004; 113(2):361-367.

(15) Ramsay, D., Mitoulas, L., Kent, C., et al. The use of ultrasound to characterize milk ejection in women using an electric breast pump. J Hum Lact 2005 Nov; 21(4):421-8.

(16) Ramsay, D., Kent, J., Owens, R., et al. Ultrasound imaging of milk ejection in the breast of lactating women. Pediatrics 2004; 113(2):361-367.

(17) Vetharaniam, I., Davis, S., Soboleva, T., et al. Modeling the interaction of milk frequency and nutrition on mammary gland growth and lactation. J Dairy Sci 2003 Jun; 86(6):1987-96.

(18) Daly, S., Owens, R., Hartmann, P. The short-term synthesis and infant regulated removal of milk in lactating women. Exp Physiol 1993; 78: 209-20.

(19) Kent, J., Mitoulas, L., Cregan, M., et al. Volume and Frequency of Breastfeedings and Fat Content of Breast Milk Throughout the Day. Pediatrics 2006; 117:387-395.

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