The Ultimate Guide to Breastmilk Storage & Handling

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Are you new to pumping and wondering about best practices for safely freezing breastmilk? This post gives you the run-down on everything you need to know about the safe storage and handling of human milk. 

Did you know human milk is alive? Well, technically only parts of it are alive. Breastmilk is more than just calories, protein, carbs, fats, vitamins and minerals. In addition to containing enzymes, growth factors, hormones and prebiotics, it also contains living immune cells, stem cells and beneficial bacterial cells (source). 

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What’s more is that human milk contains perfect ratios of micro- and macronutrients needed for optimal infant growth. The same can’t be said of straight cow’s milk, which contains much more protein than an infant’s kidneys can safely handle. This high protein load can also irritate a baby’s gastrointestinal tract, which can cause bleeding. Regular cow’s milk is also low in several micronutrients, such as iron and vitamin C, and it is low in the appropriate types of beneficial fat, which is important for brain development.

(By the way, this is why cow’s milk should not be given to infants younger than 1 year old, and why infant formula is so important! It is the safest and healthiest alternative to human milk there is, and it is much better for baby than regular cow’s milk or homemade formulas that contain it or evaporated milk.)

The ultimate guide to breastmilk storage (freezing breastmilk, collecting, storing, thawing and warming)

When you think about it, human milk is pretty neat. But honestly? Even if it wasn’t, it still takes precious time to pump it! So although we don’t “pay” for human milk the way we do for other food, it still represents a meaningful investment of time and effort. And like any food, it can become contaminated if it’s not handled and stored properly. So let’s talk about how to protect our investment, shall we? 


When pumping, it’s important to keep your environment, your pump parts, and your hands clean. This is especially important if you have a medically fragile infant. But even if you don’t, it’s still a good idea to keep things as clean as possible to maximize the storage life of your milk. As I mentioned before, human milk contains living cells that will help minimize microbe growth in your stored milk (this is why human milk can safely be left at room temperature for roughly twice as long as infant formula or cow’s milk). But after a while, too many microbes in your milk (even if they are normal, run-of-the-mill microbes on your hands or pump parts) will reduce it’s storage life. Plus, germs are gross! 


Sanitization is an important part of the storage and handling of human milk, and it is different from cleaning/washing. Cleaning/washing involves using soap and hot water (not just rinsing) to physically remove microbes, while sanitization kills or inactivates microbes–it doesn’t remove them. This is why it is important to clean infant feeding items before you sanitize them.

Breastfeeding parent washing bottles and pump flanges

What should be sanitized?

Milk collection bottles and pump parts should be washed and sanitized before their first use. Pump parts to wash and sanitize include the flanges (a.k.a. breast shields), connectors (if they’re not attached to the flange), valves and membranes (or duckbill valves, depending on what your pump uses), and backflow protectors (if your pump uses them). Typically pump tubing does not require washing or sanitization unless they come into contact with breastmilk, but check your pump’s instruction manual see how to proceed if this happens. Some pump manufacturers will have you simply replace tubing (rather than wash it) if this happens.

breast pump parts to sterilize

While the official recommendation is to sterilize pump parts after every use, the CDC states that at it is acceptable to sterilize pump parts at least once daily (source). Many parents who pump regularly will put their pump parts in a clean Ziploc bag or container and store them in the fridge for 24 hour or less, taking them out to use them as needed during that timeframe. However, if your baby is premature, under the age of 3 months, or has a weakened immune system due to illness or medical treatment, it is recommended to sterilize pump parts after every use. Speak with your baby’s pediatrician if you have questions about how often you should be sterilizing. 

countertop steam sterilizer for keeping bottles and breast pump parts sanitized

Note: If your baby is bottle-fed human milk, bottles (including teats/nipples and collars) should be cleaned/washed and sanitized prior to their first use. From then on, bottles should simply be cleaned/washed (not just rinsed) after every use. You do not need to sanitize bottles daily unless your infant is premature, under the age of 3 months, or has a weakened immune system. Don’t forget to also wash (and occasionally sterilize) brushes and sponges that you use to clean your milk collection bottles and pump parts.


There are three main methods of sanitizing your pump parts and collection bottles: Steam sterilization, boiling, and sanitizing liquids/solutions. 

Steam Sterilization can be done with a countertop steam sterilizer (like this one), in a microwave with a reusable sterilizer (like this one), in the dishwasher (if your dishwasher has a sanitize setting), or in microwave steam sterilizer bags (like these disposable ones or these reusable ones). Steam sterilization works by heating water to the point of creating steam (as the name would suggest). The temperature required to do this is hot enough to kill or inactivate most microbes, and steam effectively penetrates difficult-to-reach crevices and openings. 

Options for sanitizing pump parts
The top three items (Ziploc bag, reusable silicone storage bag, and tupperware) are useful for storing used pump parts between uses for 24 hours. The bottom three items are microwave sterilizing options. The yellow/white Medela sterilizing bags can be used a few times before being discarded, while the gray Evenflo silicone sterilizing bag and white/blue microwave steam sterilizer can be reused repeatedly.

Alternatively, if your infant feeding items are safe to boil, you can place them in a covered pot of water, bring that water to a boil, and let the items boil for at least five minutes to sanitize them. 

When you’ve sanitized your items, use a clean pair of tongs to set them on a clean, dry towel or paper towel and allow them to air dry. Hand-drying them may result in the transfer of microbes to your newly sanitized pump parts, so air-drying is preferred. 


Once your pump parts and collection bottles are clean, you are ready to start pumping! But once your milk is expressed, what should it be stored in?

Human milk can be safely frozen in any of the following:

  • Plastic milk storage bags designed for human milk storage (Lansinoh bags are a popular choice, although Nuk bags are my personal favorite, as they feel thicker/sturdier).
  • Reusable silicone bags designed for human milk storage (such as these or these)
  • Glass
  • Hard-sided clear plastic containers without BPA (avoid plastics with the recycle symbol 7)

Some parents opt to freeze their milk in food-grade silicone ice-cube trays (like this one) and then transfer the milk (which is frozen in 1 oz. portions) to a freezer safe container. This can help avoid waste if you know baby won’t finish an entire bag of milk. 

Note: Some parents will pump their milk directly into the bag they plan to freeze. The Kiinde system allows you to do this. There are also milk storage bag adaptors for certain pumps designed precisely for this purpose.

Containers for safely freezing breastmilk

Human milk should NOT be frozen in:

  • Ziploc-style bags NOT specifically designed to store human milk
  • Bottle liner bags

While it is tempting to save money on Ziploc snack bags that look just like breastmilk storage bags, these bags are more likely to break down and leak when the milk is reheated. Breastmilk storage bags are sterile and made of a thick plastic designed to protect your milk from more rapid degradation in the freezer over time. 


Depending on where you look, different sources will offer different guidelines for how long milk can safely be frozen or left at room temperature. So which one is correct?

The answer depends on a few different factors, such as how clean your milk collection process was, how warm “room temperature” is in your home, and how long the milk was in the fridge or at room temperature before you froze it. 

As a general rule of thumb, it’s easy to remember the 6-6-6 rule: 6 hours at room temperature, 6 days in the fridge, and 6 months in the freezer. The more detailed table below outlines the full milk storage guidelines I recommend following:

human milk storage guidelines

Note that once milk has been thawed, it should not be refrozen. However, Frozen milk that has started to thaw but still has ice crystals can be safely refrozen. Frozen milk that has thawed and is still cold can be stored in the fridge for 24 hours before it should be discarded. 

Additionally, note that once baby’s mouth has been on a bottle of milk, any leftover milk from that bottle should be discarded, just like infant formula. This is because baby’s mouth introduces bacteria to the milk in quantities that multiply rapidly. That said, some lactation experts agree that if baby is healthy, it is probably safe to offer the leftover milk at the next feed (within 1-2 hours). Anecdotally, many breastfeeding parents do this, and some save the milk longer than 1-2 hours. However, there are no studies on the safety of this practice, and it would not be recommended if your baby is sick or has a compromised immune system.  

Once baby's mouth has been on a bottle of milk, any leftover milk from that bottle should be discarded. It may be safe to offer it at the next feed within 1-2 hours.
It may be safe to offer milk that wasn’t finished at the last feed at the next feed. However, the safety of this practice hasn’t been studied and it is UNSAFE to do this with infant formula.


A common question pumping parents wonder is if it is safe to freeze milk from two different pumping sessions together. Absolutely it is! To do this, you’ll ideally want to make sure that any milk you plan to combine into the same container is the same temperature before you combine them. In other words, if you plan to combine freshly pumped (body temperature) milk with milk that is already cold in the fridge, allow the freshly pumped milk to get cold in the fridge before you combine it. This prevents the older, colder milk from being rewarmed by the newer, warmer milk.

Anecdotally, many parents (myself included) have combined freshly pumped (body temperature) milk with milk that is already cold in the fridge before freezing it.  

Parent putting pumped milk into the refrigerator


  • Freeze your milk in appropriate portion sizes. This usually means 3-4 ounces (learn why here). Alternatively, you can freeze your milk in 1 ounce cubes, as described above.
  • If you decide to fully fill your milk bags before freezing (not recommended), remember that milk expands when it’s frozen, so remember to leave some room in your bag to accommodate this.
  • Freeze your milk flat, so it is easy to stack/store:
Frozen breastmilk stored flat
  • Note the date and time your milk pumped, as well as whether or not you or baby was ill or were taking any medications, on the bag/container before you freeze. Breastmilk freezer bag markings may not be a reliable indicator of how much milk is actually in the bag (especially if you freeze them flat and since milk expands when frozen) so be sure to also note how many ounces is in each bag before freezing.
  • If you are freezing milk combined from multiple pump sessions, date the entire bag with the date of the oldest pumped milk.  
  • Don’t worry about mixing “day” milk and “night” milk. While it is true that milk produced at night contains higher concentrations of the sleep-inducing hormone melatonin (source), it is not clear that this has significant clinical implications. In other words, milk produced at night is not likely to be the magic solution that gets baby to sleep longer. 
  • Store milk in the back of the freezer or refrigerator, where there is unlikely to be temperature changes from the doors being opened. 
  • Use oldest frozen milk first.
  • It is normal for milk stored in the fridge (or milk thawed in the fridge) to separate. The fat will rise to the top. Don’t be alarmed if your milk does not have a thick layer of fat. This can be very normal. It used to be thought that shaking human milk might break up or denature important milk proteins, but there is no evidence to support this theory. It’s okay to shake or swirl the milk in order to mix the fat back in.
A bottle of milk that has been in the fridge for a while will begin to separate.
This milk has been sitting in the fridge for several hours. Note how the milk at the bottom is starting to look more watery, while the milk in the middle still looks normal (for now) and a thin layer of fat is accumulating at the top. If you are concerned your milk isn’t fatty enough, I encourage you to read this post.

READ MORE: How to Make Your Breastmilk Fattier (And What Doesn’t Work)


Human milk should never be thawed or warmed in the microwave, as this can result in uneven heating that can create hot spots capable of burning baby. Plus, microwaving or heating human milk at high temperatures will degrade the immunological components of human milk.

Instead of microwaving, thaw human milk in one of the following ways:

  • In the fridge (preserves the most fat content of the milk)
  • In a bowl of warm (not hot) water
  • Under warm (not hot) running water
Thaw human milk under running water or in a bowl of warm water or in the fridge.

Human milk can safely be warmed in a bowl of warm (not hot) water or in a bottle warmer.

Note: Warming human milk in bottle warmers or water hotter than 40 degrees Celsius (104 degrees Fahrenheit) may degrade certain immunological properties of the milk (source). However, I’d encourage you to not stress about this. This milk is still healthy/safe to consume and is still recommended to offer to baby before using infant formula. 

Once milk is thawed, it can be stored in the fridge for 24 hours before it should be discarded. 


If you have an abundance of frozen milk, consider donating it! You can donate it to a human milk bank or donate it privately. There are many groups online dedicated to facilitating human milk sharing/donation, such as Eats on Feets and Human Milk for Human Babies (search these groups with the name of your state on Facebook and you’ll probably find a donation group). 

Many donors will request that milk bags are traded in exchange for the milk (bag replacement) and it is normal and appropriate to honestly disclose any medications you’re taking or health conditions you have to potential donees. There are many parents who have babies that cannot tolerate infant formula well who will be very grateful for your donation, and they can pasteurize your milk at home if they so desire (you can learn more about at-home pasteurization here). 

Boost Your Milk Supply & Keep It Up

Our FREE guide will give you 5 simple, proven tips to boost your milk supply and increase your pump output. Get results in as little as just a couple days!

If you enjoyed this post, be sure to check out our digital library of helpful resources that quickly teach how to avoid common breastfeeding problems and give you the peace of mind and confidence you need to meet your breastfeeding goals.

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