Can You Combine Breast Milk From Different Days?

A question I’ve been getting more and more is whether it is okay to combine breast milk from different days. Here’s some background on where that question is coming from, and what different sources tell us.

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Mixing breast milk across days

There are a lot of rules about storing and feeding breast milk – guidelines on how long it can be kept at different temperatures, rules against refreezing thawed breast milk, and cautions against mixing warm and cold milk.

A question that I’ve been asked frequently is the following about mixing breast milk from different days:

I read this article, and it says that you can’t combine breast milk from different days? Is this true? Why would this be?

I think this is a good question. The linked VeryWell article says:

It is not safe to add breast milk that you pumped today to a container of breast milk that you pumped yesterday or last week.

What does this mean, exactly?

The article says it is okay to mix milk you pumped on the same day, but not different days. Does this mean milk pumped more than 24 hours apart?

Or does it mean that if you pump at 10pm on a Saturday and 1am on a Sunday, as is typical for an exclusive pumper, that that’s not safe to mix? (The implication being that breast milk is affected like Gremlins by the hour of midnight, or something.)

What is the evidence for this guideline?

The VeryWell article doesn’t explain WHY this wouldn’t be safe, so I checked the two cited sources to try to find the explanation.

One was the CDC, which says nothing about not mixing milk from other days. (On the contrary – it actually talks about how to mix milk from different days.)

The other is a scientific article from 2013 that is behind a medical journal paywall; a reader with access was kind enough to send me a copy. It also does not say anything about mixing milk from different days.

Evaluating the recommendation

The guidelines on mixing milk that come from the Academy of Breastfeeding Medicine Protocol 8 are as follows:

• All of the milk that is mixed is within breast milk storage guidelines, and
• You treat all of the mixed milk as if it was pumped at the time of the first expressed milk, and
• The milk you’re mixing is the same temperature (adding warm milk to cold milk can allow bacteria to grow more quickly)

Given the lack of evidence for this guideline, my guess is that VeryWell’s recommendation comes from a concern that moms won’t, in practice, treat all of the mixed milk as if it was pumped at the time of the first expressed milk?

If that’s the case, it’s unhelpful and paternalistic, in my opinion. Women are smart enough to understand nuance, and setting forth guidelines not based in evidence just makes milk storage more confusing.

Personally, I’ve combined milk from different days many times. I had read that this was safe based on what I’d read on Kellymom when my first baby was brand new:

Milk from different pumping sessions/days may be combined in one container – use the date of the first milk expressed.

Leave any questions on mixing milk across different days in the comments!