Breastfeeding: The science of milk production

Why doesn't this type of feeding work? Why do lactation experts recommend feeding babies responsively… and why is there so much emphasis on feeding within the first hour of birth? How long does milk really take to come in, and is it true that a few milliliters of colostrum are enough for a newborn?

In this video, we explore how milk production really works – and why public health guidelines today make the recommendations they do.

Here at UNICEF, we know that breastfeeding can be challenging. And we believe that, with support, breastfeeding can be possible. This is why we advocate for governments, societies and systems to provide more support so that women who want to breastfeed to be able to breastfeed.

Humans have organs so key to our existence that their Latin name – mamma – came to define the whole group of animals to which we belong. These are the mammals, defined by our mammary glands – the milk producing organs often used to feed our young.  

If you’re a mother who wants to breastfeed, knowing how they work is crucial to both establishing your supply – and to protecting it during your breastfeeding journey.  

Breasts come in all different shapes and sizes. Even in the animal world, number and location vary greatly. Unlike all other mammals, human breasts become permanently enlarged with fatty deposits after puberty.  

In pregnancy, the hormone progesterone rises and tells the body to start transforming the fatty breast tissue into a milk-making system. It happens quickly. In a typically developing pregnancy, breasts are ready to produce milk as early as the 16th week!  

But the body is clever: those high progesterone levels say to get ready to make milk, but not to actually make it… yet.  

What changes?  

After birth, oxytocin - sometimes called the "cuddle hormone" - rises. Oxytocin also is released when a baby starts suckling on the nipple.  

This oxytocin release signals to muscle cells in the breast to expand and contract - which squeezes the tiny, balloon-like structures that hold the milk, the alveoli.  

This pushes the milk through each milk duct and out through the nipple in the ‘milk ejection’ or ‘let-down’ reflex. But milk itself doesn't come in right away. At first, there’s a nutrient-rich "pre-milk" called colostrum. Often thick and yellow, it comes out slowly, partly so a baby can get used to the process of breastfeeding.  

There's not very much, but a newborn's tummy is tiny too – holding just a small amount of liquid at a time! This is why they eat very little and as often as every hour.  

Colostrum also packs a punch in terms of antibodies, helping to build up a baby's new immune system. Each time a mother feeds her baby, a signal is sent to the brain to produce another hormone: prolactin.

Levels of this hormone have been steadily rising during pregnancy. But its actions were blocked by the high progesterone and oestrogen levels. When these hormones plummet after delivery, prolactin kicks into gear, signalling to the alveoli to produce milk.

This results in milk 'coming in', around three or four days after birth.

After a feed, breasts might feel softer. But this doesn't mean they are empty of milk. In fact, about 30 per cent of the milk supply always remains within the breast!  

You can think of it like an ice machine: when ice comes out, more goes in.

This demand and supply system is regulated by prolactin, which spikes after every feed, and by the prolactin receptors on the cells lining the alveoli. If breasts empty more frequently, they fill more frequently – and more receptors are produced. In other words, each feed places an 'order' for the next feed.

But, if breasts are emptied less often, less receptors are produced, and fewer 'orders ' are put in, which can lead to a lower milk supply. It can also raise the risk of a breastfeeding journey ending earlier than a mum might want.

This is why experts say that feeding responsively, according to a baby's hunger cues, is so important, whether day or night. More than anything else, breastfeeding is a demand-and-supply system – only by feeling demand, does the body know to create supply.  

Here at UNICEF, we believe that, with the right support, breastfeeding is possible. This is why we advocate for governments and societies to provide more support – so that women who want to breastfeed can breastfeed.