Breast milk it is often described as having two parts, foremilk and hindmilk. Foremilk is the milk released at the start of a feeding; hindmilk is the milk released at the end.
Until recently, foremilk was thought to contain less fat and fewer calories than hindmilk. Consequently, foremilk was described as thin and runny, and hindmilk was described as thick and creamy.
When you compare samples of foremilk and hindmilk expressed during the same feeding, it’s true that the foremilk usually has less fat and fewer calories than the hindmilk. However, if you compare a sample of foremilk expressed at one feeding with a sample of hindmilk expressed at another feeding, the foremilk may actually contain more fat than the hindmilk.
Fat content varies with milk volume—the smaller the milk volume, the higher the fat content. Feeding patterns determine milk volume. The longer the period of time between feedings, the greater the milk volume and the lower the fat content. When the time between feedings is short (1–2 hours), babies get smaller amounts of high fat milk. More time between feedings (3 or more hours) causes babies to get larger amounts of low fat milk. Most babies get the nutrients they need to grow by following their own unique feeding pattern.
Breast fullness is a measure of fat content. The fuller the breast, the lower the fat content of the milk and the greater the difference in fat content between the milk obtained first and the milk obtained last. The emptier the breast, the higher the fat content and the lesser the difference between the milk obtained at the start of the feeding and the milk obtained at the end.
One of the amazing things about human milk is its ability to change from month-to-month, week-to-week, day-to-day, feeding-to-feeding, and from start to finish during a single feeding. To ensure that your baby gets all the fat and calories she needs to grow, breastfeed at the first sign of hunger and continue to breastfeed until your baby is full. Remember, it's important to always watch your baby, not the clock, for signs of hunger and fullness.