Understanding milk supply can be…well, confusing. We hear “supply and demand” constantly, and while that’s definitely part of the equation, it’s not the whole story. There are hormones. There’s milk removal. There’s your baby’s ability to transfer milk. There’s your body. There’s your baby’s body. And sometimes there are medical or physiological factors thrown into the mix just to keep things interesting.

Basically, your boobs have a whole production team working behind the scenes, and unfortunately, nobody handed us the employee handbook.

First up: colostrum. When you’re pregnant, your breasts begin producing milk during the second trimester. This first milk is called colostrum.

Colostrum looks and behaves a little differently than the milk you may be used to seeing. It’s thicker, often yellow or golden in color, and produced in relatively small amounts. But small doesn’t mean insignificant. Colostrum is concentrated and packed with immune factors, proteins, and other components specifically designed for your brand-new baby. Your baby’s stomach is tiny, too so those small, frequent feedings during the first few days? Totally normal. Your baby’s stomach capacity gradually increases as those first days and weeks go by while milk production is also transitioning. (bfmed.org)

Sometimes parents worry about “running out” of colostrum if they’re nursing another baby during pregnancy, leaking colostrum, or collecting colostrum before delivery. The good news is that your body isn’t working with one tiny bit of colostrum that you have to carefully ration until the baby arrives. 

During pregnancy, hormones, particularly progesterone, help keep your breasts in the colostrum-producing stage rather than switching over to full-volume milk production. You may notice your milk supply decreases as pregnancy progresses if you’re nursing another child, and that can be a normal part of pregnancy. So yes, your body is already making milk. But it also knows there’s a baby still in there waiting for the grand opening. 

Then the placenta leaves the chat. Once your baby is born, the placenta is delivered shortly afterward. And this is a pretty important moment for lactation. The placenta produces a lot of progesterone during pregnancy. Once the placenta is delivered, progesterone levels drop dramatically. That hormonal shift helps your breasts move from producing mostly colostrum into producing larger volumes of milk. 

This transition is referred to as lactogenesis II, and it typically happens during the first few days after birth. You know the feeling people describe as their milk “coming in”? This is the process they’re talking about.

Of course, hormones don’t get to take all the credit. Frequent and effective milk removal is happening at the same time, helping tell your breasts, “Hey, we have a baby here. Keep this production line moving.” 

And this is one reason retained placental tissue can sometimes contribute to delayed or very low milk production. If placental tissue remains after delivery, progesterone may remain elevated and interfere with the normal transition into higher-volume milk production. (pubmed.ncbi.nlm.nih.gov)

So, but what about all those hormones? Let’s stay on the hormone train for a minute because hormones don’t completely leave the chat once the placenta is gone. 

Prolactin is one of the major hormones involved in milk production. During pregnancy, prolactin levels rise and help prepare your breasts for lactation. After birth, breastfeeding or pumping stimulates prolactin release, which supports milk production.

Then there’s oxytocin, the hormone responsible for the milk ejection reflex—or what most of us call “let-down.” Oxytocin causes the tiny cells surrounding the milk-producing structures in your breasts to contract, helping move milk toward the nipple. So when you feel that familiar tingly, pulling, or sudden leaky feeling? Oxytocin may be clocking in for work.

And because apparently two hormones weren’t enough, estrogen, progesterone, cortisol, insulin, and growth hormone are also involved in the larger picture of lactation. Your body really said, “Let’s make this as complicated as possible.”

Hormonal or medical conditions can sometimes interfere with lactation. Certain medications, breast or chest surgery, thyroid or other endocrine conditions, retained placental tissue, insufficient glandular tissue, and other factors can contribute to low milk production. This is important because true low milk supply is not always a matter of simply feeding more or trying harder. Sometimes you can be doing everything you reasonably can and still have difficulty making enough milk. That doesn’t mean you failed. It means there may be more going on. 

An IBCLC and your healthcare provider can help look at the bigger picture, including milk removal, milk transfer, medical history, medications, hormonal factors, and your baby’s feeding patterns, and help you come up with a plan that makes sense for you. 

And then…your body starts taking notes. During the first few weeks after delivery, hormones and milk removal are working together to establish your milk supply. But as lactation becomes established, your breasts increasingly take over more of the day-to-day regulation of milk production. Think of those first few weeks as your body gathering data. 

Baby is eating this much? Okay. Milk is being removed this often? Got it. We seem to need approximately this much? Excellent. 

Your body starts getting better at matching milk production to what is being removed.

This is also why your breasts may suddenly feel softer, you may leak less, or you may stop waking up feeling like you’re carrying two watermelons on your chest.That does not automatically mean your supply has dropped. Sometimes it means your body has simply gotten better at regulating itself. Your breasts don’t need to feel painfully full for you to have milk. 

So what happens when milk isn't removed effectively? This is where supply and demand really start to matter. If milk isn’t being removed frequently or effectively, your breasts receive the message that quite as much milk may not be needed. Over time, milk production can decrease.This is why some parents notice a change in supply around the time lactation becomes more established, often during the first several weeks postpartum.

Six weeks gets talked about a lot in breastfeeding circles, and it’s a common time for parents to come in with supply concerns. But there isn’t a magical six-week button inside your boobs that says, “Okay, ladies, production cutbacks start Monday.” It’s more that hormonal regulation is shifting and your milk production is becoming increasingly dependent on milk removal and local feedback within the breast.

So…supply and demand? Yep. It’s real. But it’s a little more complicated than “baby eats → boobs make milk.”

The simplest way to think about it is: Milk removal tells your breasts that more milk is needed. Outside of underlying medical or physiological factors, more frequent and effective milk removal generally signals your breasts to keep producing milk.

Think about an automatic ice maker. You take some ice out, the machine gets the message that ice has been removed, and it starts making more. Leave the ice sitting in there, and eventually the machine gets the message that the bin is full and slows down production. Your breasts work somewhat like this, although they are obviously a lot more sophisticated than the ice maker in your freezer. Milk is being made continuously, and when milk remains in the breast regularly, local signals can slow the rate of milk synthesis. When milk is removed, those signals change and the breast gets the message to keep producing. 

So yes, supply and demand matters. A lot. But it isn’t the only thing that matters. Milk production is a combination of hormones, breast development, milk removal, milk transfer, your baby’s feeding patterns, and sometimes maternal or infant health factors. And if your supply isn’t what you hoped it would be, that does not automatically mean you didn’t try hard enough, didn’t nurse often enough, or somehow “failed.” Sometimes breastfeeding is straightforward. Sometimes it requires a little troubleshooting. Sometimes it requires a whole team. And sometimes your feeding journey looks completely different from the one you pictured. None of those things make you a better or worse parent.

Understanding how milk supply actually works can take some of the mystery, and hopefully a little of the guilt, out of the process. Because your boobs are already doing a pretty incredible amount of work. They deserve a little credit.