Mixing Breast Milk – Common Questions

Breast milk storage can be confusing – especially when it comes to mixing breast milk. Can you mix thawed and fresh milk? What about mixing milk from different sessions? Here’s what you need to know.

This post may contain affiliate links, which means that if you click through and make a purchase, I’ll be compensated at no additional cost to you. I only recommend products I love! More info here.

Can you mix milk from the different sides of the same pumping session?

Yes, there are no issues with this.

Just pour from one bottle or bag into the other, and store. Label the milk with the day you pumped it, and use it within the breast milk storage guidelines.

Can you mix milk across pumping sessions?

Yes, you can. You just want to do the following:

• Make sure that the temperature of the milk that you’re combining is all the same. (Usually, this will mean it’s all chilled.)
• Treat the combined milk as if it was all pumped at the time of the first expressed.

Let’s talk a little more about each of these.

Mixing milk across temperatures

The Academy of Breastfeeding Medicine states the following in their Clinical Protocol:

Freshly expressed warm milk should not be added to already cooled or frozen milk, to prevent rewarming of the already stored milk. It is best to cool down the newly expressed first before adding it to older stored milk.

Why is this?

One reason that we have breast milk storage guidelines is to limit the amount of bacterial growth in the milk that we feed to babies.

Keeping milk cold in the refrigerator or freezer slows this bacterial growth down. However, adding warm milk to milk that’s already in the fridge will warm the chilled milk, and allow bacteria to grow more quickly.

Note: If you’re planning on feeding the milk immediately, it’s fine to combine different temperature milk, because the bacteria won’t have any time to grow. This guideline around mixing same temperature breast milk applies to milk you are storing for later.

Treat all the milk as if it was all pumped at the time of the first expressed

This is ensure that bacteria isn’t given too much time to grow in the oldest milk.

Let’s go through an example.

Say that you pump at 10pm on Tuesday, 1am on Wednesday, and 6am on Wednesday, and then you combine all of that milk into one pitcher or bottle.

You’d label the combined milk and treat it as if it was all pumped at 10pm on Tuesday, because you need to use the first date/time that milk was expressed.

More on combining breast milk across days here.

Is mixing frozen breast milk okay?

Yes, you can. Let’s talk about mixing it to freeze, and mixing it to feed.

Mixing breast milk to freeze

Before you freeze breast milk, you can mix milk from different sessions. Just follow the guidelines we discussed above, regarding mixing same temperature milk and labeling with the date of the first expressed.

Then can you freeze it and treat it like any other frozen breast milk.

Mixing frozen breast milk to feed

You can also take thawed frozen milk from multiple containers and combine it for feedings.

Mix thawed milk with fresh milk before feeding is also okay. As discussed above, make sure to either the temperature is the same, or that you’re feeding it immediately.

How does mixing cereal with breast milk work?

When it’s time to start solids, if you want to feed baby rice cereal or oatmeal, you can mix breast milk into the cereal instead of water. Just stir breast milk into the cereal until it’s the desired consistency.

You can also use breast milk to thin out pureed solids.

Breast milk storage guidelines still apply when your milk is added to solid foods.

Per the CDC:

Don’t put cereal in a bottle. Putting infant cereal in your baby’s bottle will not make him or her sleep longer and could increase your baby’s risk of choking.

What about mixing breast milk and whole milk?

This is fine to do. Prep the bottle of breast milk and add the desired amount of cow’s milk.

(I did this to transition my baby off of breast milk. I started with mostly breast milk and just a splash of cow’s milk, and slowly increased the amount of cow’s milk in the bottle.)

Is mixing breast milk and formula okay?

Yes. Prep the formula on its own, and then combine it with breast milk. Don’t mix the formula with breast milk until you’ve already prepared it on it’s own.

When handling the mixed milk, make sure to follow both the timelines on the container of formula in terms of how long it can be out before being discarded, as well as the relevant breast milk storage guidelines for your milk.

More on mixing formula and breast milk here.

Hopefully this answers any questions that you have about mixing breast milk! Let me know if you have any questions in the comments.

References