Just like the father in this cartoon, many of us feel confused and perplexed when it comes to understanding our breasts and our babies and whether everything is OK or not. But knowledge is power! When you understand how your body builds and runs your milk factory, you’ll have a head start on solving your own personal puzzle. Even if you already know a lot, chances are you’ll find even deeper insights in this up-to-date, research-based information about milk production.

Season of breast Development: Growing a Milk Factory

Human milk production your milk factory is an amazing process that is built upon the foundation of mammary gland (breast) development that began before you were born. 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. Somewhat like a fruit tree in winter with only a few leaves and dormant buds, the non-lactating breast has large and small branches called ducts and ductules along with a small number of leaves, the alveoli,  that contain the milk-making cells. Each of the several main branches, the ducts, together with its smaller branches, the ductules, and leaves the alveoli, form lobes that intertwine yet function somewhat independently of each other.

During pregnancy, the color of the areola and nipple deepen, and the areola often enlarges in size. Small bumps called Montgomery glands, which may circle or dot the areola and are often unnoticeable until pregnancy, also grow in size. The veins along the surface of the breast become more prominent, appearing both larger and bluer. Internally, alveoli multiply and grow, filling out the lobes inside the breast like a tree leafing out in springtime. You may notice your breasts growing larger or becoming firmer as this happens. Breast tenderness is common and a positive sign that all is working normally. If growth occurs quickly and is substantial, reddish or purplish stretch marks may develop that later fade over time.

When baby is born, milk production kicks into high gear, and the tree bears its crop of fruit, the milk. The milk-making cells may continue to multiply according to the demand for milk during the next several weeks, causing additional breast growth. As baby grows older and his need gradually diminishes, the milk-making parts of the tree begin to slow down and recede. This is the autumn season of the breast, when there is still milk-making activity, but at a lower level. At weaning, the breast gradually returns to its resting winter state, awaiting a new season of pregnancy.

The driving forces underlying this cycle are hormones. From puberty on, the waxing and waning of estrogen and progesterone during your menstrual cycle slowly, over time, develop the ducts and alveoli further. This subtle stimulation continues until about age thirty-five. Pregnancy causes a much larger rise in both of these hormones, as well as the production of prolactin, human placental lactogen (hPL), human chorionic gonadotropin (hcG), and growth hormone, which all help to stimulate glandular growth. Changes in breast size during pregnancy are most closely related to the concentration of human placental lactogen, which is produced only by the placenta and so only during pregnancy. This makes your placenta a crucial player in the construction of a good milk factory for baby. Other key hormones that play a role in stimulating mammary development include insulin, cortisol, and thyroid hormones.

Anatomy of the lactating breast

Hormones And the milk-Making Process

Understanding how your hormones function helps you know if they are helping or hindering your milk supply. Each hormone has one or more types of corresponding receptors located wherever its influence is needed. The two must bind together like keys that fit only into locks that are configured for them. There must also be a good match between the amount of hormone and the number of receptors; 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 can change and is influenced by various factors. In addition, the ease in which hormones and receptors bind together can also vary; a well-oiled lock turns easily, but a rusty one does not. Type 2 diabetes is one example of a rusty lock problem that occurs when insulin receptors resist binding to insulin; we call this insulin resistance. Pregnancy and lactation cause many changes in the number and binding ability of hormone receptors important to breast development and milk production. Because receptors are equally important, hormone levels provide only half the picture. Yet, since hormones are easy to measure with simple blood tests, this is usually all we look at to judge how well our hormonal systems are functioning. It’s not as easy to measure the number of receptors because this would require taking an actual tissue sample.

Many hormones are involved in lactogenesis, the process of making milk. Like ingredients in a cake recipe, some hormones play minor roles while others are crucial for a good result. Prolactin, the major milk-stimulating hormone, is normally present in small amounts in our bodies, but its level gradually rises quite high during pregnancy, peaking at birth. The only reason that the breast doesn’t make lots of milk at this point is because the placenta is producing high levels of progesterone, which interferes with prolactin’s receptors on the milk-making cells and prevents the prolactin “keys” from having much effect. Instead, colostrum, your first milk, is produced. This earliest phase of milk production during the second half of pregnancy is known as lactogenesis I.

Since the placenta makes the majority of the progesterone during pregnancy, progesterone drops quickly once the placenta comes out, allowing prolactin to start doing its work. Within thirty to forty hours, the change to full milk production begins, and mothers usually notice an increase in milk volume between the second and fourth day. This milk is lighter in color, thinner, and more watery than colostrum. When this happens, lactogenesis II has begun, and women say that their milk has “come in.” Full milk production requires prolactin, insulin, and cortisol.

This gradual increase in milk production is a perfect match for your baby’s slowly expanding stomach. During his first day of life, he may take as little as one-half teaspoon (2.5 milliliters) or as much as 4 ounces (120 milliliters) of colostrum. After the second day, intake usually increases rapidly to between 13 and 29 ounces (384 and 858 milliliters) by the end of the first week, eventually settling to an average of 25 to 27 ounces (750 to 800 milliliters) per day. This amount stays about the same from the end of the first month until baby begins solids and needs less milk. Even though he may take more some days than others, especially during growth spurts, he doesn’t need increasing amounts of milk overall because his rate of growth slows down 50 percent by four to six months, making the amount he takes at one month enough for when he is older as well.

Milk Ejection: nature’s delivery system

Every time baby suckles, nerves in the nipple and areola send messages to the brain that trigger the pituitary to release oxytocin. This hormone causes muscle-like cells around the alveoli to contract and squeeze milk down the ducts for delivery to the baby. This process of releasing milk is called milk ejection, often referred to as let-down. Without this reflex, little milk can be removed, and when not removed well, the breast receives the message to cut back on milk production. Milk ejection is a critical component of the big picture of milk production and works extremely well the vast majority of time. The few situations that can negatively affect milk ejection and lead to lower milk production are explored in BLOG 10.

Milk ejections are not onetime events but rather can be triggered multiple times during a breastfeeding or pumping session. 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 quickly, pulling away in anger or frustration. Over time and with positive experiences, baby learns to trust the breast, while his mother learns to trust that her milk will flow. Both begin to relax into a confident breastfeeding relationship. When milk production and flow are low, some babies may remain distrustful until the flow is improved, compounding the difficulty in getting breastfeeding going.

Oxytocin release that stimulates milk ejection is unique in that it is not controlled exclusively by sensory (touch) stimulation but can also be triggered by thoughts and feelings. Many breastfeeding mothers can attest to this when a crying baby in a shopping center triggers a let-down and they notice wet spots on their shirts!

One other interesting aspect is how milk ejections affect the appearance of your milk. You’ve probably heard about foremilk, the low-fat “skim milk,” and hindmilk, the high-fat, creamier milk at the end of a feeding. These terms make it sound as if the breast produces two kinds of milk, but that’s not the case. As milk is made, the fat globules stick to the sides of the alveoli where the milk is stored and are flushed out gradually with milk ejections. The milk your baby receives in the beginning has less fat and a higher percentage of lactose, an important milk sugar, but the longer he feeds on a breast, the more cream he receives, ending up with a nicely balanced meal.

How Your Body Decides How Much Milk to Make

So how do the breasts figure out how much milk to make? Some mothers seem to initially make enough for two babies, while others start off with a lower volume that gradually increases to meet baby’s needs. At the same time that baby’s suckling triggers the release of oxytocin for milk ejection, the pituitary gland also releases a surge of prolactin to encourage continued resupplying of the breast. This is necessary because the body is constantly clearing out hormones from our system. In the weeks following birth, prolactin decreases until it reaches a lower plateau, but the more often prolactin surges, the higher the average circulating level, or baseline prolactin, will be. At the same time, receptors for prolactin multiply. The development of prolactin receptors in the breast is believed to be related to the frequency of early suckling stimulation and milk removal: the more often baby breastfeeds in the first days and weeks after birth, the more receptors are made.Good receptor development is critical to sustaining long-term milk production.

When prolactin is dropping and its receptors are being established, the milk-making process changes from being largely hormone driven (en docrine) to more locally controlled in the breast (autocrine), responding to baby’s demand by adjusting the rate of milk production up or down according to how much and how often milk is taken out. The goal of the autocrine process is to fine-tune, or calibrate, milk production to meet baby’s actual needs, with a little to spare. Lactation consultant Catherine Watson Genna, B.S., IBCLC, explains that like the marketing research department for a factory, your body spends the first few weeks after baby is born determining whether it needs more or fewer assembly lines to meet baby’s milk needs. In essence, all the early experiences of how often baby nurses and how much milk he removes is part of the body’s “market research phase” for calibrating your milk production. The more milk that you remove during this time, the higher your milk supply will be calibrated. This is critical in developing the final blueprint for a milk-making factory that will ultimately meet baby’s needs. 

Baby Calls the Shots

The breast works by a process of demand and supply: baby suckles when he wants milk, the breast delivers it via milk ejection, and then more milk is made to replace what was taken. If baby has taken all the milk and keeps asking for more, additional milk will be made. Calibration is the body’s process of figuring out how much milk to make and is designed to be an infant- driven system. This is why women may have very different breastfeeding and milk production experiences from one baby to the next. Each baby provides a new set of cues that triggers production, setting into motion a new and unique experience. Similarly, the larger-producing si can change from one baby to the next. Interestingly, mothers of baby boys tend to produce more milk than mothers of baby girls due to the fact that boys seem to grow a little faster and need a little more, thus creating a bigger milk supply in their mothers. The ability of a woman’s body to respond to baby’s milk-making signals depends not only on baby having free 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.

The Resource-efficient Breast

Your body does not like to waste precious resources. It works very hard to find a good balance of milk production that is enough for baby, plus a safety cushion, by sensitively the degree of fullness of the breast. The “golden rule of milk production” is that the emptier the breast is kept, the harder the body works to Your body does not like to waste precious resources. It works very hard to find a good balance of milk production that is enough for baby, plus a safety cushion, by sensitively the degree of fullness of the breast. The “golden rule of milk production” is that the emptier the breast is kept, the harder the body works to restock and the higher the rate of production. If a factory’s warehouse keeps emptying and orders are pouring in, management will hire new workers and speed up the production lines. Your breast tissue may actually grow, adding new machinery. Of course, these adjustments require a little time when baby is asking for more milk than is available. On the other hand, if the breast’s warehouse is consistently overflowing, management may begin to downsize the factory by reducing the number of assembly lines and laying off workers.

This happens because concentrations of a whey protein called the feedback inhibitor of lactation (FIL) increase as the breast fills up. The more FIL rises, the slower the breast makes milk, much like we slow the water down in a bathtub that is filling up too quickly. Concentrations of FIL are low when the breast is empty, telling the breast to produce milk faster, just as we turn the water faucet on high when the bathtub is empty and we want to fill it up.

If demand appears to have dropped permanently due to little or no nursing or pumping, management may even close the factory completely. We call this process of the breast permanently getting rid of some or all of the workers and equipment involution. For a period of time, this downward trend of milk production may be reversible, but after a while it is not, depending on individual factors. This is why a breast that becomes engorged needs to have milk removed on a regular basis; it is possible to go from too much to very little or no milk in a short time. This is also why it isn’t a good idea to “wait until the breast fills up” before feeding baby. More milk might build up initially, but ultimately milk production will slow down, and less will be made in the end.

Even when the baby seems to have removed all available milk from the breast, if he continues to suckle, he will get whatever milk is being made and released at the moment. It’s as if the bathtub drain is open but the spigot is on. However, milk flow is greatly reduced when there is little accumulated milk in the breast. Even though the rate of milk production is high, thirsty babies often become impatient and fussy when the milk flow is slower. Mothers frequently misinterpret this scenario and conclude that their breasts are empty and not giving milk. More likely, there is a constant production of milk—it’s just that the milk is being delivered slower than baby desires.

The Role of storage Capacity

The maximum amount of milk that can be stored before the breast says “stop!” is your storage capacity and is determined by the amount of fully developed glands inside the breast rather than exterior breast size. Storage capacity, the warehouse for the milk factory, can change and may increase during the first few months depending on baby’s appetite. Storage capacity is also one of the biggest factors in determining how often babies need to feed. Babies of mothers who have smaller warehouses need to feed more often, while babies of mothers with large-capacity warehouses have the option of eating larger meals less often.

Why Does One Side Produce More than the Other?

One of the most common observations of nursing mothers is that one breast makes more milk than the other. In fact, it is rare for two breasts to produce exactly the same amount of milk. More often, one side, usually the right, makes significantly more milk than the other. While baby can certainly create a larger supply on one side by his preference, this happens even in women whose breasts are equally stimulated by exclusive pumping. If it bothers you, try to alternate starting breasts so that both receive equal stimulation. Or, you can even try starting on the lower producer every time to even production out. But don’t be surprised if one continues to be a “super producer” It’s perfectly normal.

Does Milk Production Have an Upper Limit?

Unfortunately, we don’t have the complete answer on upper limits of human milk production because researchers have not yet explored this subject matter. In theory, according to the demand-and-supply principle of lactation, the more milk that is removed, the more milk should be made. However, ultimately there are limiting constraints. Some women’s capabilities are limited because their bodies didn’t develop a lot of basic glandular tissue during puberty. Some started with enough, but for some reason that tissue didn’t blossom well in response to pregnancy hormones. Others simply don’t have enough functional ducts and nerves because of previous breast surgery. Still, why doesn’t the functional tissue that exists just produce higher and higher levels of milk when prompted to do so? Part of the answer may lie in the fact that milk supply is calibrated according to the amount and frequency of milk removal during the early weeks, within the limitations of over- all breast tissue. Some women seem to have great flexibility and can easily increase milk production with additional stimulation for quite a while after delivery, while other women seem to have a short window of calibration, so that their milk production reaches a plateau and is not as responsive to demands for more milk. Apparently, these women’s breasts begin losing some of their “leaves” earlier than others do.

The Milk Supply Equation

The components necessary for good milk production can be summed up in this equation:

The Milk Supply Equation Sufficient glandular tissue

+ Intact nerve pathways and ducts

+ Adequate hormones and hormone receptors

Adequately frequent, effective milk removal and stimulation


If one part of this equation isn’t there, milk production can be compromised. If more than one part falters, the possibility of problems is greater. Sometimes boosting another area of the equation can compensate for an area that is missing or low, though not always, because each one of these factors is necessary for good milk production.

Milk Production Is Designed to Succeed

We hope you now have a good understanding of how your breasts make milk and how you and your baby influence the process of milk production. As you read through the next Blogs, keep in mind that nature designed lactation to be a robust process with multiple safety nets. If one part of the milk-making factory isn’t functioning optimally, another will work harder to compensate. This is why most mothers around the world breastfeed very successfully even under circumstances that are less than ideal. It is only when there are numerous breakdowns in the system or one problem is particularly severe that mothers and babies have difficulty breastfeeding. In most cases, keeping mother and baby together whenever possible so that they can respond to each other’s cues will be key in getting things back on track. But if you are feeling confused about your baby’s behavior or what’s happening with your breasts, the next BLOG 2 will help you determine what’s normal and what’s not.

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