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Most mothers who breastfeed know that the composition of their milk changes after the first few days, from the nutritious colostrum that your breasts create in the two or three days after birth to the true milk that they make after that period. Many mothers also know that breast milk can vary a bit from day to day with their diet.
But did you know that the composition of your milk changes during a feeding? That your milk is richer at the end of a feeding than it was when your baby started? As your baby feeds, the amount of fat in your milk goes up. It may contain 1% fat at the beginning and 5% or more by the end. The highest concentration of fat is seen in your milk after your baby has been feeding for 30 minutes.
But Why Does It Change?
Before we answer that question, let’s look into the anatomy of a human breast. Milk is produced in lobes within your breast. You have between five and 20 of them. The lobes are made up of smaller parts called lobules that actually make the milk. Milk is carried to the nipple through ducts and builds up in your breasts between feedings, which is why your breasts become engorged if you haven’t breastfed in a while.
When your baby starts to suck (and sometimes before!) your breasts let down, which means that your milk is squeezed down the ducts to the nipple. But the fat in your milk tends to cling to the sides of the lobules and some of it lags behind at the start of the feeding. As the feeding session progresses, this fat starts to be expressed, which gradually increases the fat content of your milk later in the feeding. If you feed your baby very frequently, there is less difference in fat content.
So unlike cow’s milk in a bottle on the supermarket shelf, the milk your breasts produce is not homogenized into a product that has a uniform fat content. Some people call the milk your breast expressed early in the feeding “foremilk” and the milk that he or she gets later in the feeding “hindmilk.”
So What Does It Mean?
The changing levels of fat in your breastmilk is why you need to let your baby finish nursing on one breast. According to La Leche League International, this is why you don’t need to make sure you use both breasts during a feeding.
It is more important to let baby finish the first breast first, even if that means that he doesn’t take the second breast at the same feeding. Hindmilk is accessed gradually as the breast is drained. Some babies, if switched prematurely to the second breast, may fill up on the lower-calorie foremilk from both breasts rather than obtaining the normal balance of foremilk and hindmilk, resulting in infant dissatisfaction and poor weight gain. In the early weeks, many mothers offer both breasts at each feeding to help establish the milk supply.
La Leche League International. Common Breastfeeding Myths.
Bonyata K. Foremilk and Hindmilk – What Does it Mean?