There are a lot of rules when it comes to breast milk storage guidelines. It can be confusing trying to make sense of it all. So when it comes to mixing milk, you can be sure there are rules.

It makes sense to many parents that when you have a bottle half full of refrigerated breast milk from an earlier pumping session, you should be able to add warm breast milk from a recent pumping session.

The thought process is that it saves space, supplies, and freshly expressed breast milk added to cold breast milk that is also fresh should be okay. However, it’s not. Breast milk needs to be at about the same temperature before you add warm milk to any other container of freshly expressed breast milk.

Why can’t you add warm breast milk to cold? This article will answer this question and break down more rules when it comes to mixing breast milk with formula and more. We’ll also discuss recent changes to the feeding and storage guidelines for breast milk, which may surprise you, and provide you with answers to why you can’t add warm breast milk to cold breast milk.

What Happens When Cold Milk Meets Warm Milk

If you’ve been wondering: Why you shouldn’t add warm breast milk to cold. The answer requires a bit of a science lesson. Or at least it did, until very recent changes.

Suffice it to say that bacteria multiply and thrive in warm-temperature milk. Bacterial growth occurs more rapidly in warm breast milk or room temperature freshly expressed milk, while chilled milk stored in the refrigerator is slower to grow bacteria. Frozen breast milk stays good for so long because the bacterial growth process has been, well, frozen.

Any time you have a rapid acceleration in temperature, you have a breeding ground for bacteria to thrive. So when you combine fresh breast milk that is body temperature with older stored milk that is cold or cooled, the cold breast milk gets warmer faster, creating rapid bacterial growth.

The Great Debate: Pooling Milk

The American Academy of Pediatrics (AAP) updated its milk storage guidelines in 2021, stating that they see no actual harm in mixing warm and cold breast milk. While this has some moms’ heads spinning, since previous guidelines have warned against this practice, the AAP is a trusted source of information for handling breast milk and many other facets of childcare.

While you should always combine your current warm milk with the freshest milk that is chilled, they say you can add warm milk to your cold milk with little risk.

This is music to the ears of exclusively pumping moms, who rely on their breast pumps and storage methods to feed their babies. The okay on mixing warm milk with cooler milk means less work, less worry, and fewer supplies needed.

However, there is still nothing wrong with being a bit hesitant or preferring to stick to older practices.

Feeding Bottles of Similar Temperatures

Now that we know the AAP’s take on the issue, we will discuss how to mix milk that began at different temperatures, with the wish to get them to the same temperature before mixing.

While you can, based on the findings of the AAP, mix warm milk with cold, other parents may not wish to do so and don’t want to fight bacteria worries at all. The following are the methods of getting milk bottles to the same level of coolness or warmness before mixing.

How to Combine Fresh Breast Milk with Cold Breast Milk

If you want to add warm breast milk that you have just pumped to cold breast milk stored in the refrigerator, you need to get both quantities of expressed breast milk to the same temperature.

You can do this in two different ways.

Warm Breast Milk Up

You can extract the cool, freshly expressed milk from the fridge and bring it to the same temperature as the freshly pumped milk that has just been pumped. Ensure that both the warm and cold milk are from the same day. You can put the cooled milk in warm water to get it up to the same temperature as the warm milk.

Once both of the breast milk storage containers are at the same temperature, you can go ahead and combine them. You are now adding warm milk to more warm milk.

Remember that any time you warm breast milk, you need to feed it to your baby immediately. You cannot take cold milk, warm it, combine it, and then put it in the freezer later to have frozen milk. Once the milk has been warmed up, you have to use it by feeding the milk immediately to the baby.

Cool It Down

You can also combine milk if you store expressed breast milk in the refrigerator to cool it down to the same temperature as the cooled milk. Once both storage containers are cooled, you can make one bottle by combining cooled milk pumped from both sessions on the same day. It is ready to go if your little one doesn’t mind drinking cold milk.

When You Should Not Mix Milk

Frozen breast milk, cold milk, warm milk, or milk from different pumping sessions of different temperatures or times should not be combined if your child has any health complications, is or was a preemie, or is ill.

This information is pushed by the Center for Disease Control and the Academy of Breastfeeding Medicine. Any infant with special needs or health complications should be given fresh milk, or you should freeze breast milk in a properly sealed container immediately after pumping.

This is because ill babies and preemies do not always have the immune systems needed to deal with any risk of bacterial growth at all, and it is best not to introduce the risk. Storing breast milk when you could combine it can be a pain, but when it comes to human breast milk, it is best to stick to all of the rules to guarantee the safest feeding methods for your baby.

Discover the ultimate on-demand breastfeeding classes designed to help you overcome common breastfeeding hurdles and challenges. Learn everything you need to know about breastfeeding, from latching and positioning to low milk supply and nipple soreness and so much more! 


Stacy Stewart’s expert guidance will help you enjoy breastfeeding and navigate your journey with confidence. Join now and give your baby a great start to life.

Can you mix breast milk and formula?

So, we’ve answered: Why you can’t add warm breast milk to cold.

Now, you may be wondering: Can you mix breast milk and formula? The answer to this question is much more straightforward: yes.

Always start by preparing the infant formula. Instructions are included on the can if you are mixing powder formula with breast milk. After you have mixed the desired amount of formula, it’s time to add the breast milk.

Cold Milk

There are no absolute rules against giving babies bottles of cold milk. However, don’t be shocked if the baby refuses to eat it. You can mix cool or cold formula with cold milk, thaw frozen milk to the same temperature, and then feed the baby as usual. Just make sure you combine milk that is not spoiled or sour with the formula, and never dilute it with water.

How to Mix Warm Milk with Formula

You can combine warm breast milk with infant formula that is also warmed. This is often done by mothers who have trouble maintaining milk supply so that their little one is still getting the benefits of breast milk and getting enough to drink. It’s a wonderful option for the mother trying hard to breastfeed but isn’t making enough milk to sustain their little one’s needs.

Frozen Milk

You now know that you should not combine warm and cold breast milk. But what are the rules when it comes to previously frozen milk?

As long as you properly thaw breast milk and have it properly stored in a freezer bag or tightly sealed container, it can be warmed up using the same methods you would use to warm up cool milk.

You can set it out on the counter to get thawed milk, which you can then run under warm water, which will warm it up enough for you to start combining milk in formula form. You can also use a bottle warmer on thawed milk to warm it up a little faster, and then put the mixture into the baby’s bottle and feed as usual.

How long can you leave breast milk out?

Depending upon the beginning temperature of the milk, when left out, you can leave breast milk out for various lengths of time.

Room Temperature

Milk that has been freshly expressed and is at room temperature can be left out for up to four hours before it needs to either be dumped or immediately frozen and kept in freezer bags in a deep freezer. Make sure to label any milk you plan to store with a label, marker, or post-it note that includes the date and time it was placed in the freezer.

Refrigerator

Milk in the refrigerator that has not been removed, frozen, or warmed up can stay there and be safely fed to your baby for up to four days. After this period, the milk will either need to be dumped or immediately frozen.

Deep Freezer

After being freshly pumped, milk that has been properly frozen is good in the freezer for up to six months. Older milk is still okay, but the milk begins to lose its nutritional value after six months.

The CDC and most other health institutes and lactation consultants will generally tell you that frozen milk in a deep freezer is good for up to 12 months. However, if you plan to donate your breast milk, many organizations will not take it if it is older than six months.