How you make milk


It’s not just your breasts that are involved in making milk. There is a set of systems in your body that help you to produce milk. 

In this section, we’ll break down the systems and hormones involved in lactation. We’ll also talk about what can increase and decrease how much milk you make. 

Here are the topics: 

🍒 Your breast systems

🥛How your body first starts to create milk

❤️How hormones are involved in creating milk

📈What causes your milk supply to go up or down

❔Common questions

🤔Why are we focusing so much on this when we should be talking about pumping? Because understanding what is going on in your breast is important for understanding how to best set yourself up for pumping. 

Your breast systems

We already talked about your breast anatomy. In this section, we’ll talk about how that anatomy helps you to make milk. 

If you look at just one alveoli (little hollow grape-like sac), you will see the three kinds of systems your body uses to make milk: alveolar cells, myoepithelial cells and your capillaries. 

  • Your alveolar cells line the inside of the alveoli (sac), and they create and secrete your milk into the hollow sac. When your alveoli are full of milk, your breasts feel full and you’ll really want to nurse or express milk to empty them. 
  • Your myoepithelial cells surround your alveolar cells. They are smooth muscle and they contract (when oxytocin hits) to push your milk out along your milk ducts towards your nipple. This is what is happening when you feel a letdown. 
  • Your capillaries surround these cells. They bring blood which contains hormones and nutrients to the receptors which are on the outside edge of your alveoli. 

The histology (microscopic structure) of your breast when you’re making milk is totally beautiful. 

👆Microscopic structure of alveoli containing milk from the Michigan Histology and Virtual Microscopy Learning Resources

👆Microscopic structure of a milk duct from the Michigan Histology and Virtual Microscopy Learning Resources

For a guided tour around these beautiful images, take a look at this video (sound on) to see your alveoli, your milk ducts and other parts that make milk in your breast. 

How your body first starts to create milk

There’s some fairly complicated biology happening in your body when you start lactation, but it really boils down to three main phases.

They’re all called Lactogenesis. Lacto, meaning milk, and Genesis, meaning creation. 

Here’s a summary. 

Lactogenesis I - Secretory Differentiation

While you are still pregnant, in the last few months before birth, your milk cells are producing the first milk thanks to the hormones in your placenta. This first milk is called colostrum. 

Colostrum tends to be thick and yellow-ish and contains all sorts of nutrients and antibodies that your baby needs. 

When you first express milk, whether it be in the week or so before birth, or in the days after, it will start as colostrum. 

Lactogenesis II - Secretory Activation

After birth, when your placenta is ‘delivered’ from your body (vaginally or through c-section), your body has to balance its hormones again. 

There is a rapid drop in progesterone, which starts the process of making mature milk. The receptors on your milk cells are switching out progesterone for prolactin. 

What’s prolactin? Keep reading.

Your milk is now ‘transitional’, meaning that it is a mix of the colostrum that’s left, and the mature milk that is being created. 

This transitional milk can last a few days to just over a week. 

Lactogenesis III - Galactopoiesis

At the same time that you deliver your placenta, another process starts alongside Lactogenesis II (secretory activation), called galactopoiesis. 

This phase is just about maintaining your mature milk. To do that, your prolactin levels need to stay high, which comes from frequently “removing” your milk and from frequently stimulating your nipple and breast. 

In practice, it just means that you need to nurse or pump frequently to keep making milk. If you want to make more, you need to remove more. If you want to make less, you need to remove less. 

How hormones are involved in creating your milk

Prolactin and oxytocin are the two main hormones that directly impact your milk production. 

Both already exist in your body, and both are secreted from the pituitary gland, deep inside your brain. They each have different pathways, which means they are triggered in different ways. 


Let’s start with prolactin, which is the one that makes the milk. 

As we mentioned before, prolactin already exists in your body. After birth, you’ll see a big increase, with some people increasing faster and higher than others. 

You don’t need to be a breastfeeding mother to make it. Here’s a case of a woman, who had already been through menopause, who established breastfeeding

Prolactin and your nervous system

To understand how prolactin works, you need to understand a little bit about your nervous system. One of the functions of your nervous system is to take information that you sense around your body, process it, and trigger reactions.

Too vague? Fair enough. Let’s imagine that you touch a hot stovetop. The heat creates pain signals that travel back to your central nervous system (your spinal cord and brain) that cause you to pull your hand away. 

In the case of feeding, your baby sucks at your nipple, which sends a message through the nerve (called T4) that runs from your nipple to your spinal cord up to your brain. Your brain then secretes prolactin to your blood, which runs all the way back through your body and to your breast to create milk (pro - lactin = pro - milk!). Cool, huh?

How to increase prolactin if you’re not directly nursing your baby 

Now, it doesn’t have to be a baby that sucks at your nipple for this message to be sent and responded to. 

Theoretically, it could be a pump or your hands. It’s just that a baby is very good at stimulating the nipple by latching, and also good at stimulating the other nerves in your breast with their hands and body. 

👇Have you ever seen a breastfeeding baby massage or play with their mother’s breast like this?


If you’re trying to increase your prolactin, and you are able to, you could bring your baby to your breast. Even if they’re not feeding, they’re still helping to send that message back to your brain along the nerves in the nipple (T4) and breast (T3 and T5) to release prolactin. 

This is particularly important in the first few days as your body transitions to making mature milk. 

And remember, that while stimulating your nipple is the most important, stimulating around your breast (above and below the areola) also works. 

Increase prolactin by stimulating your breast frequently

Now as with all things in the human body, it’s never that simple. Your prolactin response and levels aren’t the same every time you stimulate your nipple. One study showed prolactin levels rising at about 25 - 30 minutes into a feed, and dropping off between feedings. The longer the period your baby is away from your breast, the less prolactin you have. 

This is supported by other studies that show that more frequent nursing (10x per day) resulted in more weight gain for babies at Day 15, than less frequent nursing (7x per day), even though the total time nursing was the same (137-138 minutes). They found no difference between the groups at Day 35 though, so perhaps this is only important in the first few weeks. 

Unfortunately, again, we couldn’t find any studies looking at the effect on prolactin from pumping (as opposed to nursing). 

Prolactin levels over the course of the day

You may have noticed that you collect the most milk at night or early morning sessions. This may be because prolactin levels tend to be higher during the night and early morning. 


This hormone is also sent from the pituitary gland (deep in your brain). 

When oxytocin hits the myoepithelial cells around your alveoli (the grape looking parts that create and store milk in your breast), they contract, and push the milk out through your milk ducts and out your nipple. Some people can feel this contraction - it’s called a letdown. 

Oxytocin does this outside the breast to other smooth muscles too. You may have felt your uterus also contract when you get a letdown.

Remember how prolactin is released in response to a trigger? Oxytocin is not too different, it just has slightly different triggers. There are three main ways that your body triggers a release of oxytocin:

  1. Hearing your baby (or any baby) cry, smelling them, looking at them. Like Pavlov’s dog, you can start to become conditioned to release oxytocin, and push milk stored in your breast out your nipple from just seeing your baby. For some people, this can be quite a forceful rush of milk. For others, it can be very subtle. 
  2. Nipple stretching. Your nipple being stretched as your baby feeds or your pump works. 
  3. Your baby using their hands to massage your breast. 

Unlike prolactin, oxytocin can be affected by immediate stress or shock. But stress doesn’t mean that you can’t make milk. Plenty of people breastfeed all around the world in very difficult situations. Even new motherhood alone can be inherently stressful for people and they are able to breastfeed. 

Other hormones

Some research suggests that there are many (up to 20) hormones that are found in your gut that are affected or changed during lactation. These hormones influence the health and wellbeing of you and your baby, but we won’t go into them too much here. Hormones are super complicated, and although they’re super interesting, knowing more about them is not necessarily going to change how you pump. You can read more about the gastro-intestinal hormones here

What causes your milk supply to go up or down?

This is complicated. There are many factors that can affect how you develop and maintain your milk supply. 

The main factor that influences milk supply is how frequently you remove milk from your breast by pumping or nursing. 

You may have heard people talk about how milk production is about “demand and supply”. They just mean that the more milk your baby (or pump) demands from your breast, the more your body will make.  If you think back to everything you’ve just learned about prolactin and oxytocin, this makes sense! 

If you’re worried about your supply, you’re not alone. One of the most common reasons for giving up breastfeeding is because of perceived lack of milk. Thankfully (although it may not help alleviate your worries), it appears that fewer people have true low milk supply than people who are concerned about it

We will go into more detail about the factors that can affect your supply, and what to do about it in a new chapter (watch this space). 

Factors that can reduce milk supply

For now, here are some factors that can reduce milk supply: 

  • A long time between expressing or feeding milk. In the first few months, at least 10 feeds or pumps per day is about what you need to establish supply. 
  • Pressure on your milk-making cells, for example from engorged breasts.
  • Pressure from restrictive bras.
  • Pressure from breast implants or damage from breast surgery.
  • Inadequate glandular tissue (but remember, you can’t tell how much glandular tissue you have from looking at your breast).
  • Iron deficiency. Iron creates hemoglobin, which is something your red blood cells need to carry oxygen to your milk-making cells. It can also make you really tired, which means you might pump or nurse less than you normally might. 
  • Loss of blood due to postpartum haemorrhage.
  • Conditions where you have hormone imbalances, such as hyper- or hypo-thyroidism or diabetes 
  • Being overweight or obese. In the first week after birth the speed that your body establishes lactation can be slower
  • Retained placental fragments. Wait, what on earth is that? Remember how lactogenesis II doesn’t start until your placenta is out of your body? So, if during your birth or surgery not all of your placenta was removed, then your body may not have shifted over its hormones to releasing prolactin and oxytocin you need for creating mature milk. 
  • Some drugs that you might have had to take during labor. There is an up-to-date resource for checking various drugs called LactMed, but it’s best you consult your doctor. 
  • Smoking. Smoking can affect how much milk you make. 
  • Using pacifiers or dummies. The research and breastfeeding world is conflicted on this one. Some studies say that pacifier use is associated with reduced breastfeeding but others, like this Cochrane review of 1302 babies, say that using pacifiers had no significant effect on breastfeeding rates
  • Using nipple shields. Despite a few studies, the research doesn’t seem to be settled. On one side, the shield may dull your sensation and could be associated with lower milk volume and stopping breastfeeding early. On the other hand, many women argue that the shield made all the difference in the early stages of feeding. This study has a good review of shield pros and cons.

Factors that can increase milk supply

And here are some factors that can increase milk supply: 

  • Expressing more frequently.
  • Pump both breasts at once. A study, again in Western Australia, found that “double pumping”, or pumping both breasts at the same time had faster letdowns (and more of them) than pumping one breast after the other. Why? Suction on both breasts at once may send signals to release more hormone to make milk. 
  • Warmth. Warmed shields (to just a few degrees above your body temperature) was associated with faster milk flow in one study. 
  • Compression while pumping. One study found that applying compression with vacuum resulted in 10 to 46% more milk expressed
  • Relaxation or meditation. In one study, mothers who listened to a 20 minute guided relaxation while pumping, expressed 63% more milk that the mothers who didn’t. The relaxation was about imagining a drip of water, turning into a trickle, to a waterfall that slowly turned white as milk. In another study of 30 mothers with babies in the NICU, those who had music therapy had lower stress levels and significantly higher milk production than mothers who didn’t. 
  • Galactagogues. This is a word that refers to medications, herbs, foods or drinks that are believed to increase milk production. It doesn’t mean that they do increase milk production. Some people swear by them, but unfortunately a systematic review of galactagogue studies found that there aren’t enough studies, or the studies are limited by small sample sizes, poor eligibility criteria and randomisation and can’t be validated. As a result the Academy of Breastfeeding Medicine has published a protocol saying that they cannot recommend any specific galactagogue. In addition to this, there are galactagogues that can do harm, so you should speak with your doctor about whether you should take them or not.

Common questions on how you make milk 

Have we not answered your question yet? 

Here are some common questions we hear on this topic.

How does your breast know how to make milk?

Your clever pituitary gland releases prolactin and oxytocin, mostly in response to you or your baby stimulating your nipple by suckling or pumping. Prolactin tells the glands in your breast to make breast milk. Oxytocin signals your let-down reflex to release the milk. This causes the smooth muscle around your alveoli (those grape-like glands) to contract and squeeze the breast milk out into your milk ducts. Pretty cool, huh?!

How can you increase your milk supply?

The main factor that influences your milk supply is how frequently you remove milk from your breast by pumping or nursing. It’s supply and demand - the more milk you feed your baby (either by nursing or pumping), the more milk your body will make. So if you want to increase your supply, breastfeed your baby more frequently or increase your number of pump sessions (as long as it’s comfortable for you to do so!).

What determines how much breast milk you produce? 

How much breast milk you produce is based on a variety of factors but is thought to be closely linked to your baby’s genetics. Genetics determine growth rate and metabolism. So for most people, their bodies will increase or decrease the amount of breast milk they produce in order to meet your baby's exact needs.

How quickly do breasts refill?

You actually don’t stop! You produce milk before, during, and after feedings—so there’s no need to wait between feedings for your breasts to fill up again. A long gap between feedings signals to your breasts to make less, not more, milk (see supply and demand above). 

What are the best times of the day to pump?

You may find that you pump more milk at certain times of the day. It’s not the same for everyone, but usually your milk volume is highest in the morning and lowest in the late afternoon or early evening.

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Next up: learn about What's in your milk