Can You Mix Breast Milk From Different Days?

Your Guide to Safely Storing Breast Milk

Stockpiling your breast milk means your baby can always have your milk even when you're away. But safe storage is essential.

You're working to build your breast milk stockpile, but you only have so much space available in your fridge and freezer. Maximizing storage by combining milk from different pumping sessions makes sense, but it is safe? The answer is yes. You can combine milk as long as you follow certain guidelines to cut down the risk of bacterial growth.

Rules for Combining Breast Milk From Different Pumping Sessions

It's generally safe to combine fresh milk with previously expressed and stored milk. But it's important to first chill the fresh milk, so it's the same temperature as the older milk. Keeping milk cold helps prevent bacterial growth. If you add warm breast milk to the cold refrigerated milk, it increases the temperature and can make it susceptible to bacteria. If you add warm milk to frozen milk, it can partially thaw the milk.

The following combination situations are acceptable as long as the new milk is chilled first:

  • Fresh milk combined with milk from a previous session that day
  • Fresh milk combined with refrigerated milk from a previous day for continued storage in the fridge
  • Fresh milk combined with refrigerated milk to move to the freezer for longer storage
  • Fresh milk combined with previously frozen milk for continued storage in the freezer
  • Two containers of refrigerated milk from different pumping sessions combined into one container for continued storage in the fridge or freezer

Length of Storage for Combined Containers of Breast Milk

How long you store the milk affects the nutritional value and safety of the milk. When you store milk, it gradually loses its ability to fight off bacterial growth. If you store milk for longer than the recommended times, you could risk food poisoning caused by bacterial growth. Your baby may also miss out on key nutrients that can diminish over time, so it's important to use the milk as soon as possible.

When combining milk from two different pumping sessions, always use the date from the oldest milk as your guide for determining the storage length. Put the date on the container for the original pumping session, and don't change it if you add fresh milk later.

Storage times for breast milk vary by storage type:

  • Fresh milk at room temperature in a warm room: Three to four hours
  • Fresh milk at room temperature: Four to eight hours
  • Fresh milk in the refrigerator: Three to eight days; 72 hours is optimal
  • Thawed milk in the refrigerator: 24 hours
  • Self-contained freezer compartment: Six months
  • Deep freeze unit: 12 months; six months is optimal

If you combine fresh milk with refrigerated milk from two days ago, you can keep it in the refrigerator for another one to five days instead of the full three to eight days based on the oldest milk date. If you add fresh chilled breast milk to a container of frozen milk from one month ago, you can still store it in the freezer for up to five more months.

Other Breast Milk Storage Tips

Keep these tips in mind when storing breast milk, whether you combine milk or not:

  • Always write the date on the container to avoid keeping the milk for too long.
  • Store milk near the back of the refrigerator or freezer so the milk doesn't warm up every time you open the door.
  • Store milk in small portions to avoid waste. Once your baby starts eating a serving of breast milk, it's best to use it all or discard the leftover to prevent bacterial growth.
  • To avoid contamination, wash your hands before pumping or handling breast milk. 
  • To keep contaminants out of the milk, store milk in containers that seal well. 
  • Leave space in the container if you plan to freeze the milk. The liquid expands as it freezes, so it needs room in the container. 
  • Use the oldest breast milk from the freezer first.
  • Thaw frozen breast milk by putting it in the fridge for about 12 hours. If you need to thaw milk quickly, pop it in a bowl of warm water.

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