Tips for storing, freezing and thawing breast milk

Knowing how to store breast milk is important so you can preserve those nutrients and make it last as long as you need it to. Here are some tips and recommended supplies for storing, freezing and thawing breast milk.

November 18, 2019

Everyone should know how to store breast milk. Refrigerated or frozen milk can be used when you’re away at work or when your partner wants to feed the baby, and it can also be donated if you’re approved to do so. But freezing and thawing breast milk can feel a little tricky, especially at first, so we’re going to break down what you need to know to store breast milk successfully, based on advice from lactation consultants and International Board Certified Lactation Consultants (IBCLCs). (Note that this guidance is general and for full-term, healthy babies. If you have questions, reach out to a lactation consultant.)

What supplies do I need for pumping? 

  • A manual or electric breast pump. Choose a great breast pump by reading reviews, talking to other mothers, or asking your lactation consultant. FYI, many breast pumps are also covered by insurance, so make sure to check out your options there.
  • Pump parts. If you’re working with an electric pump, make sure you’ve got your neck flanges, tubes, rubber duckbill valve, and backflow protectors (if you’re using a closed-system pump, like the Spectra)
  • Bottles or storage bags. According to Dr. Sears, there isn’t much research about how storage containers affect human milk. So you should choose based on what’s most convenient for you and what your needs are. Your options include:
    • Plastic bottles. Make sure to choose containers that are certified BPA-free. For any reusable containers, check for dishwasher safety, and make sure they can be frozen and heated. 
    • Glass containers. Glass or hard-sided plastic containers provide the best protection for nutrients and immunities. Another benefit of using glass containers is that once your little one is no longer nursing, they can be used for baby food. Talk about reducing your carbon footprint! 
    • Bags. Note that if you’re planning to refrigerate or freeze your milk, you might want to pump directly into bags to cut down on cleaning and save storage space. Lansinoh makes BPA-free bags that you can pump into directly.  
  • Optional: Stickers and pens. This is helpful for labeling your milk so you know when it’s from and when to use or store it. You can even use tape if you’re in a pinch or don’t want to shell out for more. 

What containers should I have around for storage? 

  • Storage container for taking your milk on-the-go. We especially like the Pack-It, which has built-in cooling and can keep things cold for up to 12 hours. Note, however, that it is not leakproof, so bag your bottles or bags in a ziplock bag to keep things from leaking through in an accident. 
  • If you’re on the go, we recommend packing extra ziplock or milk bags in your pumping bag. This can help in a variety of situations; if you accidentally forget your cooler bag, you can fill one with water and freeze it or get ice to keep your milk cool. Or, if you need to take messy pump parts home, you can toss them in the bag. 
  • Milk bags (if you pumped into bottles). These are made by multiple brands. 

What containers should I use for cleanup? 

  • Bottle brush and soap: First, you’ll want to clean the bottles and pump parts that come into contact with your breast milk. 
  • Sterilizer or large pot: You can sterilize your parts by using specially made steam sterilizer bags or by placing your parts in a pot of water and boiling them for five minutes.
  • Cleaning wipes and spray: Medela makes convenient quick-clean wipes so you can wipe things down on the go. They also have a new spray that also does the trick. 

How long does breast milk last in the fridge or freezer? 

At room temperature (up to 77°F), breast milk can be stored for up to four hours. It can be refrigerated up to four days and frozen within four days of pumping. Use breast milk storage bags, clean glass, or air-tight, hard-plastic, BPA-free containers.


How long can I wait before refrigerating? 

Remember: Breast milk is milk. So you can think about freezing and thawing breast milk the same way you do whatever milk you drink. 

Whether you’re refrigerating or freezing, it’s best practice to store your breast milk as soon as possible after expressing. Although breast milk can remain at room temperature (60–77°F) for approximately four hours, after that, the healthy fats and nutrients in the milk will start to break down, and it will start to become less beneficial to your baby. This is because an enzyme called lipase begins releasing fatty acids to prevent bacterial growth as soon as the milk is expressed. By storing breast milk immediately, you’ll slow this process down.


How do I know if I should freeze my milk?

You should check with a lactation consultant on maximizing storage potential. If you don’t think you’ll use milk soon after expressing it, you might want to go ahead and freeze it to preserve nutrients. 

According to Paula Yount at Mother-to-Mother, it can be possible to use all the storage options in succession for healthy full-term babes.

For example, store freshly expressed breast milk at room temperature for up to 10 hours, THEN in a refrigerator for up to 8 days, and THEN in a freezer for 3–12 months. 

Where in the fridge should I be storing milk?

If you’re storing breast milk in your refrigerator, place it in the back of the fridge above the vegetable compartment. This is generally the coldest part of the refrigerator, and the temperature is less likely to fluctuate than the front of the shelves or in the door when the fridge is opened throughout the day.

Remember that refrigerated breast milk needs to be kept at 39°F or colder for no more than three to five days. 

Can I mix body-temperature milk with cold milk?

When you’re learning how to store breast milk, it’s important to understand how temperature affects the milk. 

Although expressed milk can be refrigerated or frozen right away, you shouldn’t mix cold milk and body-temperature milk together. This can cause the cold milk to break down and introduce bacteria. You can, however, combine milk that’s from the same temperature if you wish.

Similarly, frozen breast milk that has been thawed should not be refrozen. Once you thaw your milk, or bring refrigerated milk to room temperature, plan to use it or toss it. And if your baby doesn’t finish a bottle, the safest approach is to throw it away, but research is mixed on this question.  

What size container should I use? 

Once stored breast milk has been opened and exposed to the air, bacteria can begin growing, and the milk can spoil. If your baby doesn’t finish an entire portion of milk, you will need to throw it away. Because of this, try using small individual containers to store breast milk in your refrigerator or freezer. We recommend storing milk in 1–4-ounce portions. This increases the number of containers/bags you need but ultimately tends to lead to less waste of breast milk. 

When freezing, how much of the container should I fill? 

Anyone who is researching how to store breast milk will quickly learn how freezing and thawing breast milk compares to refrigerating it. In the freezer, breast milk must be stored at 0°F or colder. It can remain frozen for up to six months in a standard freezer that’s connected to a fridge or up to 12 months if the milk is stored in a deep freezer.

Additionally, breast milk expands when it freezes. Because of this, make sure you only fill your containers two-thirds of the way when freezing breast milk. Most bags designed for freezing breast milk have a line that you shouldn’t fill past. 

Sometimes freezer burn causes the milk to have a foul odor or taste, especially in milk from women with more lipase than others. Lipase is an enzyme that breaks down milk fat, and excessive lipase activity can cause the milk to smell bad even after short periods of freezing. Some infants will reject this milk, and others won’t be bothered by the smell or taste. 

The only real solution for avoiding freezer burn is to scald the milk after pumping and before storing. Just be sure not to boil it, as boiling can remove some of the nutrients. 

What are the best thawing and warming techniques?

There are several ways to thaw frozen breast milk. One of the easiest methods is to place frozen breast milk in the refrigerator overnight. Depending on the temperature of your fridge, the frozen milk should be completely thawed within 12 hours. Previously frozen milk may be kept in the refrigerator for up to 24 hours after it has finished thawing.

You can also hold the container under warm running water, put it in a bowl with warm water, or use a bottle warmer. Aim to use body-temperature, or 99°F, water, but never exceed 104°F when warming stored breast milk. 

Don’t thaw your frozen breast milk at room temperature, and never use microwaves or boiling water. Heating the milk in a microwave or boiling water can destroy the milk by cooking it. It can also create uneven “hot spots” that can burn your baby.

Once defrosted and warmed up, you may notice the stored breast milk has separated. This is completely normal, but you may wish to swirl or stir the milk to distribute the fats more evenly.

Yount also advises that it’s important to use your “oldest” milk first or rotate your stock of milk. A simple way to do this is to use fresh milk from the fridge throughout the week, and then once or twice a week thaw a bag or two of frozen milk. You can freeze a bag or two of freshly pumped milk to replace the frozen you’ve used. This keeps your stock “rotated” and still allows baby to have the bulk of milk be fresh milk. Fresh milk matters, too, because the composition of your milk changes as baby grows (hence why you don’t need to feed more as they get older—the milk adjusts to accommodate).

How should I label the containers after pumping?

Finally, labeling is crucial when learning how to store breast milk. Many bottles and breast milk storage bags include a blank space for labeling or come with stickers. You can also make your own labels, or use a strip of masking tape and a marker.

Once expressed into the container, label the milk with the date—and the time. This will ensure that you always know exactly how fresh it is. You can even estimate your own “expiration dates” depending on whether you choose to freeze or refrigerate the milk. 

Does time of day impact milk composition? 

When pumping, it’s important to write down the time of day, and not just for keeping an eye on its expiration. Many new mothers have no idea that breast milk is said to be different in the morning than it is in the afternoon and evening. Morning breast milk is designed to wake the baby up, while milk pumped at night helps baby fall asleep. So, when you reach for refrigerated or frozen breast milk to feed your baby, reach for the milk that was produced at a similar time of day. 

If you’re pumping and breastfeeding, check out our breastfeeding FAQs for even more helpful information. 

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