pediatriki on line: Foremilk and Hindmilk

Over the course of her breastfeeding journey, it is almost inevitable that a mother will encounter the terms "foremilk" and "hindmilk". There is certainly no shortage of confusing and conflicting advice on the concept, which has led to a lot of misunderstanding.

What do the terms "foremilk" and "hindmilk" mean?

In The Breastfeeding Answer Book, "foremilk" and "hindmilk" were defined as follows:

"Foremilk" is the milk the baby receives when he begins breastfeeding, which is high in volume but low in fat (1).

"Hindmilk" is the milk near the end of the feeding, which is low in volume but high in fat (1).

Actually, the terms "foremilk" and "hindmilk" only really refer to samples of breastmilk taken right at the beginning of a feed ("foremilk") and samples of breastmilk taken right at the end of a feed ("hindmilk"). Even then, the advantage of using these terms is debatable.

Why?

A photo on Wikipedia "helpfully" shows two 25ml samples of human breastmilk: "foremilk" on the left and "hindmilk" on the right:

The problem with showing a sample of breastmilk from the start of a feed together with a sample of breastmilk from the end of a feed is that it tells us nothing about what happens to the breastmilk in between the two samples. Mothers end up mistakenly believing that their breasts make two different types of milk, and wonder how long they must feed for before the "foremilk" magically switches over into "hindmilk" (2).

So if the terms "foremilk" and "hindmilk" are misleading and there's no "magic switch" from one to the other, what really happens?

Since The Breastfeeding Answer Book was published in 2003, research in human lactation has advanced (2), so we now understand more about how breastmilk is produced:

Your breasts make just ONE type of milk

There is no magic switch between two different types of milk over the course of a feed. In fact, there is no sharp distinction between breastmilk at the start of a feed and breastmilk at the end of a feed (3). "Milk does indeed change consistency during a feed - but this happens gradually rather than suddenly" (4).

Why does the consistency of breastmilk change over the course of a feed?

The answer lies in what happens to the fat in breastmilk when breastmilk is

produced:

Your breasts make just one type of milk, containing 'sticky' fat globules that tend to stick to each other and to the milk ducts in the breast as breastmilk is produced (2,3).

stored in the breast:

Breastmilk produced between feeds moves down towards the nipple and collects in the breast, leaving increasing amounts of fat globules 'stuck' together further up the milk ducts (3).

and emptied from the breast:

At the initial let-down, the milk at the beginning of a feed may be lower in fat because the fat globules are still 'stuck' further up in the milk ducts (5). As the feed progresses, the fat globules are flushed out of the milk ducts by milk ejections (4). As the breast is emptied and more and more fat globules become dislodged, the breastmilk increases in fat (3). This has the overall effect of a gradual increase in fat content of breastmilk in each breast independently as milk is removed.

How do we know that the increase in fat is gradual?

Studies involving several mothers have been conducted in which samples of breastmilk were taken at intervals over the course of a feed. The evidence from these studies suggests that the fat content of breastmilk increases gradually as the breast becomes progressively emptier over the course of a feed (6,7).

It's much easier to explain this gradual increase in fat content using pictures...

...so I visited The Countess of Chester Hospital, where several wonderful people enabled me to take consecutive small samples of breastmilk from one breast only over the course of one pumping session, and professionally photograph the results. The samples were then taken to the Cheshire and North Wales Human Milk Bank, where the fat content of each sample was analysed.

This is the snapshot we got of the gradual increase in fat content of my milk over the course of one pumping session:

A snapshot of my breastmilk:
At the time the samples were taken, I was tandem feeding. My youngest baby was just over 8 months old, had started baby-led-weaning, and was still about 90% breastfed.
Each test tube contains around 6ml of breastmilk.

Note the gradual colour change of each breastmilk sample. This is the gradual increase in fat content from the beginning of a feed (test tube 1) to the end of a feed (test tube 12).

Remove test tubes 2-11, and the result is a picture similar to the one on Wikipedia:

This shows a sharp contrast between the breastmilk at the very beginning and the breastmilk at the very end of a feed only - it tells us nothing about what happens to the breastmilk in between the two samples. Only the photo containing all 12 test tubes shows the complete picture: a gradual increase in fat content.

So what's that stuff that rises to the top of expressed breastmilk?

When breastmilk is left to stand for a while, it separates into two layers. Thisisn't the separation of "foremilk" from "hindmilk"; it is simply the fat in the milk rising to the top, not the separation of two different kinds of milk (4).

The reason this separation doesn't occur in a bottle of cow's or goat's milk you buy in a shop is that this milk has been homogenised.

"Whole homogenised milk is identical in fat and nutrient content to whole standardised milk however it has undergone a process known as "homogenisation" which breaks up the fat globules in the milk. This spreads the fat evenly throughout the milk and prevents a creamy layer forming at the top." (6)

After a while, the breastmilk in our test tubes separated into two layers (click on the image to open a larger version in a new window):

Note that the breastmilk in test tube 1 does, in fact, contain a layer of fat.

Note also the increasing thickness of the fatty layer that collects at the top of the breastmilk from test tubes 1-12. This further demonstrates a gradual increase in fat content from the beginning of a feed (test tube 1) to the end of a feed (test tube 12).

Again, removing test tubes 2-11 shows only a sharp contrast between the thin fatty layer that rises to the top of the breastmilk at the very beginning, and the much thicker fatty layer that rises to the top of the breastmilk at the very end of a feed.

This photo tells us nothing about what happens to the breastmilk in between the two samples. Remember: only the photo containing all 12 test tubes shows the complete picture: a gradual increase in fat content.

Showing us pretty pictures is nice, but how much fat did the breastmilk samples actually contain?

Before I continue, I must stress that this is just a snapshot of my breastmilk. It is unlikely that the results shown here will be an exact reflection of what happens to every mother's breastmilk. However, it does provide a rough idea of what happens, so here are the results of the analysis of each sample of my breastmilk:

TS = Total Solids, ie. the bulk of the rest of the milk
Protein, carbohydrate, fat and TS values are in grammes per 100ml

If looking at raw data isn't your thing, here's a line graph showing the amount of protein, fat, carbohydrates and total solids present in each of my 12 breastmilk samples (fat content is shown in red).

The graph clearly shows there was a gradual (if slightly wobbly) increase in fat content as the breast is emptied. Quite plainly, there is no arbitrary "magic switch" between two different types of breastmilk during a feed.

Remember, though, that all of this is just a tiny piece in a vast breastfeeding jigsaw...

Although it is true that the fat content does gradually increase as the breast is emptied, it is important to understand that the difference in fat content between milk at the beginning of a feed and milk at the end of a feed can vary greatly from mother to mother. Not only this, but it can vary greatly over a 24 hour period, even among individual mothers (2). The difference in fat content can also depend to some degree on the amount of time elapsed between each feed - this is another reason why it might not be a good idea to wait for your breasts to "fill up" before a feed.

The fuller the breast,

the lower the fat content of the milk
the greater the difference in fat content between the breastmilk at the beginning and end of a feed (2,3)

The emptier the breast,

the higher the fat content of the milk
the lower the difference in fat content between the breastmilk at the beginning and end of a feed (2,3)

This is not simply a case of the longer the feed, the higher the fat content: if there is a shorter gap between feeds (such as during periods of cluster feeding), the milk at the beginning of a feed may not necessarily be low in fat (2,8).

"Babies can show a wide variety of feeding patterns, suckling for varying lengths of time and at varying intervals over the course of a day, and maintain a healthy weight" (4,8)

We can and do get far to preoccupied with the issue of fat content - take another look at the graph showing the amount of protein, fat, carbohydrates and total solids present in each of my 12 breastmilk samples:

Did you spot the line labelled 'TS' (Total Solids)? That line represents all the other components of breastmilk which are important too!

The most important thing to remember is this:

In the vast majority of cases, as long as you are:

breastfeeding on demand
and allowing your baby to come away from the breast spontaneously at the end of a feed,

there is no need to worry about what's in the breastmilk your baby is getting.ALL of the components of breastmilk are essential for the growth and the physical and mental development of your baby, not just its fat content.

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Many thanks to:

Lynda Coulter (Cheshire and North Wales Human Milk Bank) for very kindly agreeing to analyse the samples of my breastmilk.

Janet Beech and Linda Gendler (Cestrian Ward, The Countess of Chester Hospital) for entertaining my mad idea and helping to co-ordinate the experiment.
Becca Williamson (Bosom Buddy breastfeeding peer supporter) for being my extra pair of hands.

Jenni Ross (Medical Photography, The Countess of Chester Hospital) for taking such excellent photos of my breastmilk samples.

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References