Layering Frozen Breast Milk in Bottles

bottles of breast milk in refrigerator
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If you want to collect, freeze, and store your breast milk, but you're only getting a little bit of breast milk each time you pump, you may be wondering if you can combine these small amounts together.

It would be convenient if you could combine, freeze, and store your breast milk in the amounts that your baby is taking at each feeding. By putting small quantities of breast milk together into one container, you can also maximize the storage space in your freezer. But, is it safe?

Freshly expressed breast milk or even breast milk that has been pumped and left out at room temperature, is warm. Warm breast milk cannot be added to already frozen breast milk.

If you add warm breast milk to frozen breast milk, the frozen breast milk will defrost a little bit. And, once your frozen breast milk begins to defrost, you should not refreeze it. It should be used right away or moved to the refrigerator and used within 24 hours. 

Adding Refrigerated Breast Milk

While you cannot add warm, freshly expressed breast milk to a container of frozen breast milk, you CAN safely add freshly pumped then cooled down, refrigerated, breast milk to frozen breast milk.

Adding more (cold) breast milk to already frozen breast milk is called layering. Layering can be done throughout the day until the storage bottle contains the amount of frozen breast milk that you would like it to have. Here are some tips for layering your breastmilk or adding more breast milk to already frozen milk. 

How It's Done

You can add more breast milk to already frozen breast milk if you collect the breast milk on the same day, you have a healthy, full-term baby, and it's for your private use at home.

Place your freshly pumped breast milk into the refrigerator and allow it to cool for 30 minutes to 1 hour before adding it to a container of frozen breast milk. By refrigerating the breast milk first, it will help to prevent the already frozen breast milk from defrosting.

When adding the cold breast milk to a container of already frozen milk, the amount that you add should be less than the amount of the breast milk that is already frozen. This is another way to help to prevent the frozen milk from defrosting.

You should pay attention to how much breast milk that you put into each container, and be careful not fill the storage container to the top. Your breast milk will expand as it freezes, so you need to leave some room at the top of the bottle to allow for that expansion. If your storage bottle is too full, it can burst in the freezer.

When adding more breast milk to your containers, keep in mind how much breast milk your baby is taking at each feeding. It will help to reduce waste if you store your breast milk in 2, 3, or 4-ounce portions.

You can always defrost another 2 to 4 ounces if necessary, but if you have breast milk stored in 6 or 8-ounce portions, you cannot refreeze what you don't use. You will have to throw away any leftover breast milk that has been thawed and warmed.

It's safe to layer (or add more pumped milk) your frozen breast milk as long as the fresh milk is cooled and refrigerated first, AND all of the milk is collected on the same day.

When Not Add to Frozen Breast Milk

Collecting on Different Days

If you collect breast milk on different days, you should not add it to already frozen breast milk. Breast milk collected on different days should be kept separate. 

Bringing Milk to Hospital

If you are pumping your breast milk to bring to the hospital, you should not add it to frozen milk especially if your child is sick or premature and in the hospital, you should not open and close the storage bottle to add more breast milk.

Each time you open and close the storage container, there is a risk of introducing germs and bacteria. This type of contamination is more dangerous for a sick or premature baby than it is for a healthy, full-term infant. So, once you seal the top of the collection bottle the first time, leave it closed until it's time to use it in the hospital.

Collecting for a Milk Bank

If you are sending your breast milk to a milk bank, follow the collection and storage guidelines that the milk bank provides.

Pumping Session Contaminated

If you are not in a clean environment, or you were not able to wash your hands before pumping, don't add the milk from that expression to a bottle of already collected breast milk. You don't want to contaminate the entire bottle that you worked hard so to collect. 

1 Source
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  1. Centers for Disease Control and Prevention. Breastfeeding: proper storage and preparation of breast milk.

Additional Reading

By Donna Murray, RN, BSN
Donna Murray, RN, BSN has a Bachelor of Science in Nursing from Rutgers University and is a current member of Sigma Theta Tau, the Honor Society of Nursing.