As a certified breastfeeding counselor, it is important you be able to answer common breastfeeding questions. “Can I mix breast milk and formula?” is a very common question many combo feeding parents have. You will of course cover this in your training.

However, even once your training is complete, there may be times you need to look up common questions. There is a lot of information to remember, and recommendations do change over time.

It can also be helpful to look up an answer so you can better explain the reasoning to clients. Helping clients make informed feeding decisions is important.

Can You Mix Breast Milk and Formula?

The short answer is absolutely. You can mix breast milk and formula in the same container. That said, there are different reasons why one might do this, and why it isn’t always a great idea.

The main reason it is typically discouraged is mixing breast milk and formula in the same container can lead to waste. Breast milk and formula have different storage guidelines. Once mixed, you would need to follow the storage guidelines for formula which does not keep as long as breast milk.

Breast Milk Storage Guidelines

Guidelines can vary slightly depending on the source. As a CBC, it is safest to provide your clients with current guidelines from the CDC or the equivalent agency in your country. They can then make an informed decision regarding which guidelines they opt to follow.

One rule of thumb some parents use is that if the milk smells bad, it may be bad even if it is within the ‘safe’ timeframe. It is important to remember that breast milk can smell soapy, or have other distinct smells, but it should not smell sour.

Freshly Expressed or Pumped Milk – Up to 4 hours room temp, 4 days in fridge, and ideally 6 months but up to 12 in freezer.

Thawed or Previously Frozen – 1-2 hours room temp, 1 day (24 hours) in fridge, and never refreeze thawed milk.

Once a baby feeds from the bottle, use leftover milk within 2 hours.

Be sure to read Breast Milk Storage Guidelines to learn more about safe storage.

Formula Storage Guidelines

Due to the composition of formula, it does not safely store for as long as breast milk. Prepared powdered formula has a short storage time in comparison to breast milk.

Once prepared, powdered formula can be stored:

• Up to 2 hours at room temperature
• Only 1 hour after baby drinks from the bottle
• Up to 24 hours in the fridge

Ready to feed and liquid concentrate formulas can be stored:

• In their sealed containers at room temperature until the expiration date
• In the fridge up to 24-48 hours after being opened
• Only 1 hour in a bottle after baby has fed from it

Why You Should Not Mix Breast Milk and Formula

As you can see, the storage guidelines for formula are much shorter than breast milk. Once you mix the two, if baby does not finish the bottle, you are then wasting the breast milk.

If baby is combo feeding, it is best to feed the breast milk from one bottle. Then, if needed, provide a bottle of formula. Anything which is left and needs to be wasted will be the formula rather than the breast milk.

Exception – When To Mix Breast Milk and Formula

The only time breast milk and formula should be mixed is if a pediatrician recommends this. In some cases, a baby may need additional calories due to prematurity or specific health conditions. In this case, powdered formula and fortifiers might be added to breast milk.

Parents should never do so without a pediatrician’s guidance as improper early diet can cause organ damage, weight issues, etc. If a client is directed to do this, encourage them to continue to ask their provider about the benefits and risks and when they should cease mixing.

In this case, the breast milk would need to follow the storage guidelines for formula.