What is transitional milk?

For the first couple of weeks of your baby’s life, the composition of your breast milk is changing dramatically. Discover the extraordinary properties of this transitional milk

No day with your newborn is the same – and the same goes for your breast milk. When your milk comes in, your breasts may grow to a size you previously couldn’t have imagined, and they’re still changing on the inside too. During the first week, the milk-making cells, and the way they connect to one another, adjust for ongoing breastfeeding.¹ From then until around the two-week mark, the milk they produce is called transitional milk.²

“With the delivery of the placenta, the mother’s level of the pregnancy hormone progesterone starts dropping rapidly,” explains Professor Peter Hartmann, a leading authority on breast milk composition from The University of Western Australia. “As progesterone falls, there’s an increase in milk synthesis and a more ‘normal’ composition of breast milk develops, although it takes a couple of weeks to become mature.”

Stages of breast milk: An interim phase

If colostrum is your baby’s ‘starter’ food and mature milk is their long-term nutrition, transitional milk is the bridge between the two.

Think of them as three different stages of breast milk, rather than three separate types. The basic ingredients remain the same for as long as you breastfeed, but their levels go up or down depending on circumstances. It’s during this transitional period that they change most, on a daily basis, just as your baby’s needs change. 

Your milk changes because it’s full of bioactive components including cells, hormones and helpful bacteria. It’s not a simple switch as your mature milk ‘takes over’. Instead, the changes are fine-tuned to fit the demands of your baby’s development.^3,4

“A major influence on the composition of the milk is the volume that the mother’s producing,” says Professor Hartmann. “When her supply is very low, her milk has a different composition to when her supply becomes higher.”

Transitional milk: Increasing in quantity

As your baby grows she quickly begins to need more food and a different balance of nutrients. The quantity of milk you produce over this period increases dramatically too: you may make a whopping 600 or 700 ml over 24 hours⁵ – compared to the tiny amount of colostrum you produced at first.

“The components of each species’ milk are specific to meet the needs of its young”

Your breasts are now in ‘build supply’ mode as they learn how much milk your baby needs. They’re becoming more mature too, as is your milk. Compared to colostrum, there are higher levels of fat in transitional milk, as well as increased lactose, a natural sugar that gives your baby energy.²

“Lactose levels go up abruptly two or three days after your baby is born,” explains Professor Hartmann. “You also get changes in the fat as the milk starts to contain more of the medium-chain fatty acids C10 and C12. As well as being a rapidly metabolised source of energy, these are thought to have anti-viral effects. Also, the sodium and chloride decrease to very low levels at this time, so that the milk has a very low salt content.”

Protein: Getting the balance right

The protein content of your breast milk alters too. There are two classes of protein in human milk: casein and whey. Casein turns into solids (curds) when it meets the acid in your baby’s stomach, and may help them feel fuller for longer. It also has antimicrobial properties. Whey is rich in antibodies and remains liquid, so it’s easier to digest – especially important for newborns. As your baby’s gut gets more resilient during the transitional stage, the proportions of whey and casein in your milk alter from around 90:10 in colostrum to 60:40 after a month (and 50:50 if you continue breastfeeding for a year).⁶

This balance of proteins is the ideal blend for humans, as our bodies grow relatively slowly while our brains become large and complex. It also delivers all the amino acids your baby needs for their brain, eyes and other organs to function healthily.

The amount of whey protein in breast milk is significantly higher than in other mammals’ milk. The proportions of whey and casein in cow’s milk are the opposite: 20:80 (that’s why it’s not suitable for babies under a year old).⁷

“Milk is purpose-specific,” explains Professor Hartmann. “Although there are certain components in all species’ milk – proteins and fats for example – when you start looking at what types of proteins are there, what types of fat, you can tell which animal it comes from. The components of each species’ milk are specific to meet the needs of its young.”

Changing protection levels of transitional milk

Although your baby is still tiny, over the first couple of weeks they've already starting to develop their own immune system and needs less immediate protection from you.

Reflecting this, the concentration of protective enzymes and antibodies within your milk changes. Some, including lactoferrin (a protective enzyme) and sIgA (an antibody), decline, while others, such as the bacteria-killing enzyme lysozyme, increase.⁸

“The protein content of the milk actually comes down around this time too,” Professor Hartmann points out. “The protective proteins are synthesised at the same rate, but they’re diluted by the higher volumes of milk being produced.”

The concentrations of the minerals zinc, copper and manganese – all of which help to support your baby’s immune system – also drop as their immunity improves.⁹

When breast milk becomes fully mature

During the transitional period, the composition of your breast milk is adjusting remarkably. By the end of the first month, your milk becomes fully mature. This means it’s suitable for your baby as they grows older. Its make-up won’t change that much again, whether you continue breastfeeding for a few months, a year, or far beyond…