How you make milk

You make milk when messages from your breast travel to your brain, which trigger hormones, which then travel through your body, to tell your breast to make milk.


Your breast systems

You need three kinds of systems to make milk:

  • alveolar cells
  • myopithelial cells
  • blood supply (via capillaries)

The alveolar cells create and contain the milk, they’re hollow and the milk is made in them.

The milk gets pushed out along the ducts to the nipple by the myopithelial cells (kind of like the uterus lining that contracts when oxytocin hits it).

The blood supply is there to bring hormones and nutrients to the receptors, which are on the outside edge of your alveolar cells.

We didn’t always know the anatomy so well. There was some comprehensive study of lactating breasts in the 1840s, but it wasn’t until 165 years later that researchers ultrasounded lactating breasts, and showed that we had misunderstood quite a few things about how your breast and baby works to feed. Read this study, and this study for their learnings from ultrasounds.


Fun fact: this ultrasound research was funded by Medela (you know, the yellow breast pump brand) - it’s nice to see a long-term contribution to research like this from pump manufacturers.


🤔 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.


4 common misconceptions you may have about your breast that could affect your success pumping

  • Your ducts aren’t evenly spaced like a clock around the breast.

You may have more ducts around some parts of your breast than another. This is why when you pump, you may get better flow in one part than another part. Also, it may explain why you may seem to experience blocked ducts in some parts of your breast, but not others.

  • You have about 3 to 5 functional pores in your nipple where your milk comes out.

This is a bit different to a common idea out there that you have 25 to 30 squirting nipple pores in your nipple.

  • Your fat isn’t sitting behind your breast, or in one particular spot.

It’s actually all mixed up with the glandular tissue in your breast. Some of that fat even comes out in your milk.

  • Although you may feel like one, your nipple and breast is different from a cow or a goat.

Those animals have lactiferous sinuses, which are storage cavities in the teat that hold milk. Humans don’t have this. Your milk goes straight from your lumen (the little grape-like looking sac that creates and holds your milk) to the nipple.


Where do these misconceptions come from?

You can be forgiven for not knowing this. A few of these misconceptions come from the early 1800s (about the time of Netflix’s Bridgerton series if your history isn’t your strong suit), when breast anatomy was being studied through dissection by two men - Sir Astley Paston Cooper, 1st Baronet and William Fetherston Haugh Montgomery.

Apart from both having very long, rather fancy names, and both having a propensity for naming parts of the female breast after themselves (Montgomery Glands and Cooper’s Ligaments) (which feels a little creepy in the modern day), they both made advances and errors in what we know and assume about the breast.

To be fair, they didn’t have ultrasounds to confirm their studies (that came 165 years later - they were injecting hot wax into milk ducts to try to define their shape.

Why it took over a century and a half (from 1840 to 2005) to really study the anatomy of the lactating breast, when we know breast milk to be critical in developing babies, we don’t know.

👆Here is a drawing from one of Montgomery’s books, looking at nipple changes during pregnancy.



Prolactin and oxytocin are the two main hormones that directly impact 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 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!).

Stimulating your nipple (and your breast) to increase prolactin

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 at stimulating your breast with their body.

There are two other nerves (called T3 and T5) that run from your breast above and below the nipple that also send the signal to release prolactin when stimulated. Have you ever seen a breastfeeding baby massage or play with their mother’s breast?

[insert gif of baby playing with breast]

So if you’re trying to increase your prolactin, you would spend time early on stimulating your nipple (T4) AND your breast (T3, T5), although the nipple is most important. Check out this study, that shows the highest level of prolactin release occurs when you stimulate nipple and breast, as opposed to just breast.

Timing of stimulation to increase prolactin

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).

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

Creating receptor sites for prolactin

In the first few hours after your placenta is no longer in your body, your hormones have a big change.

Happens early - first two hours after birth - get more receptors



Prolactin increases during pregnancy, which helps increase the mammary glands to prepare for milk production. Suckling at the nipple stimulates milk production by increasing prolactin blood levels.

There is a balance of prolactin and oxytocin necessary for producing the milk and keeping it in the breast. Oxytocin makes the cells in the breast tissue (alveoli) contract, which allows the milk to be collected in lobules. When oxycontin is produced more quickly than prolactin, the milk is released from the alveoli and travels through the ducts. Skin-to-skin contact also promotes oxytocin release. Both prolactin and oxytocin levels are stimulated by suckling at the nipples.