Breastmilk is unique to each mother and baby dyad, and varies over the course of the entire breastfeeding journey to adapt to the infant’s changing needs.
The dynamic nature of breastmilk is present over the course of the breastfeeding journey, but it also varies in supply and composition over the course of the day, and over the course of a feed.
Some use the terms hindmilk and foremilk to describe this phenomenon but this can be confusing as it does not allude to the gradual nature of the change in milk composition over the course of the whole feed.
Rather than producing two distinct milks, the breast only produces one type of milk and its composition changes over the course of each feed. Fat globules tend to adhere to the alveolar walls, and to each other. As more milk is produced in the interval between feedings, it moves towards the nipple ready for the next feed, while the fat globules remain on the alveolar walls. The milk mixes with any milk remaining from the last feed so the longer the interval between feeds, the more dilute the remaining milk will become.
With the onset of feeding or expression, rapid flow of milk out via the nipple is triggered by the milk ejection reflex. As the breast empties, the fat globules are dislodged and the fat content of the milk increases. The concentration of fat in the milk is directly proportional to the emptiness of the breast.
Foremilk refers to the milk at the beginning of the feed, while hindmilk refers to the milk at the end of the feed. Although it is presumed to be the case, foremilk does not always have a lower fat content than hindmilk. If the interval between feeds is short and baby begins the next feed on the same side, the breast may still be relatively empty and the milk remaining therein may not have been diluted and may have a higher fat content. As more milk is produced, the milk with the lower fat content could reach the baby later in the feed than the higher fat content milk at the beginning.
It is important to feed on demand according to hunger cues, and to allow the infant to drain the first breast before offering the second to take advantage of the changing nature of breastmilk over the course of the full feed.
Kent, J. C., Mitoulas, L. R., Cregan, M. D., Ramsay, D. T., Doherty, D. A., & Hartmann, P. E. (2006). Volume and frequency of breastfeedings and fat content of breast milk throughout the day. Pediatrics, 117(3), e387-395.
Kent, J. C. (2007). How breastfeeding works. Journal of Midwifery & Women’s Health, 52(6), 564-570.
Ballard O, Morrow AL. Human Milk Composition: Nutrients and Bioactive Factors. Pediatric Clinics of North America. 2013;60(1):49-74.