Foremilk is the milk (typically lower in fat) available at the beginning of a feeding; hindmilk is milk at the end of a feeding, which has a higher fat content than the foremilk at that feeding. There is no sharp distinction between foremilk and hindmilk – the change is very gradual. Research from Peter Hartmann’s group tells us that fat content of the milk is primarily determined by the emptiness of the breast — the less milk in the breast, the higher the fat content.
A woman’s breast really only makes one type of milk, the higher-fat milk that we typically think of as hindmilk. As milk is produced in the breast, the fat globules in the milk tend to stick to each other and to the walls of the alveoli (where the milk is made). Between feedings, milk collects in mom’s breasts and gradually moves out toward the nipple, leaving more and more of the fat “stuck” further back in the milk ducts. The more time between feedings, the lower the fat content of the foremilk available to baby at the beginning of the feeding.
Once the let-down (or Milk Ejection Reflex/MER) is triggered (by baby’s nursing, pumping, etc.), the milk is squeezed down the ducts until it becomes accessible to the baby. Milk production is not faster during letdown – the flow is simply faster. There are several let-downs per feed, although most mothers only sense the first one.
As the breast starts to empty, the fat globules begin to dislodge and move down the ducts (let-down facilitates this process). So the further into the feed, the higher the fat content of the milk, as more and more fat globules are forced out. The end result is that the milk gradually increases in fat as the feeding progresses.
The first water you get out of the tap isn’t usually hot, but cold. As the water runs, it gradually gets warmer and warmer and warmer. This is what happens with the fat content in mom’s milk – moms’s milk gradually increases in fat content until the end of the feeding.
Since fat content is is directly related to the degree of emptiness of the breast, it is possible, depending upon nursing pattern, for fat content to be higher at the beginning of a particular feeding than it is at the end of some other feeding.
This is how it works with mother’s milk too – the longer the time between feedings, the lower the fat content at the beginning of the next feeding. If feedings are closer together, you’re starting off with a higher fat content.
As a particular feeding progresses, fat content increases, milk volume and flow decrease, and milk synthesis speeds up. Because every baby varies in the amount of time it takes him to receive his fill of the higher-fat milk at the end of the feeding, it is important not to switch breasts while baby is actively nursing.
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Many thanks to Paula Yount of Mother-2-Mother.com for the great analogy!