Mixing Formula With Breast Milk in the Same Bottle

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Breastfeeding parents may choose to supplement their babies with infant formula for many reasons. If you do decide to supplement with formula, there are a few things you should consider before you mix formula with breast milk.

Mixing the two is possible, even in the same bottle, but you need to be careful to follow the mixing instructions before combining formula with breast milk. Improper mixing can lead to an over-concentration of nutrients that may pose a danger to your baby.

Mixing Breast Milk with Infant Formula
Verywell / Jessica Olah

Why Mix Formula With Breast Milk

There are many reasons you may choose to mix breast milk with formula. Whatever the reason may be, if you decide to give your child both breast milk and formula, there may be times when the two need to be combined in the same bottle.

Extra Sleep

Feeding pumped breast milk and formula is one way to allow parents to get some extra sleep. This way, one parent can take a turn feeding the baby during the night, giving the other one a chance to rest.

Low Milk Supply

Some parents may struggle to make enough breast milk to breastfeed exclusively. Supplementing with formula ensures that your baby gets the nutrition they need while still getting some breast milk.

Returning to Work

Many parents choose a combination of breast milk feeding and formula feeding for convenience when they return to work.

Safety Considerations

Infant formula is made to provide your baby with a specific amount of calories and nutrients in a specific volume of fluid. For example, a standard formula is 20 calories per fluid ounce. So, if you prepare the formula as directed, your baby gets the expected amount.

However, if you add powdered formula or concentrated liquid formula directly into your breast milk before you dilute it with water, it changes the balance of nutrients and water in both your breast milk and the infant formula.

When your baby is an infant, their kidneys are not yet mature. The kidneys of newborns and young infants need enough water to process all of the nutrients in their feedings, especially the proteins and the salts. When feeding is too concentrated, it can be dangerous and too much for your baby's body to handle.

Therefore, when preparing your child's formula, you should always use the correct amount of water and follow all the instructions that you are given.

How to Mix Formula With Breast Milk

When you buy formula for your baby, you will usually get one of these three types: concentrated liquid, powdered, or ready-to-feed. How you mix it with breast milk will depend on what type of formula you have.

Concentrated Liquid and Powdered

If you use concentrated liquid formula or powdered formula, be sure to make it according to the manufacturer's instructions or any alternate instructions that your baby's doctor gives you.

Mix the formula first, separate from the breast milk. Concentrated and powdered infant formulas are typically diluted with sterile water or safe drinking water that has been boiled for five minutes and then cooled.

Depending on the quality of the water in your area and your baby's health, you may be able to use tap water. Talk to your child's doctor to find out if tap water is a safe alternative.

Once the concentrated liquid or powdered formula is prepared, it can then be added to a bottle of breast milk or given after the bottle of breast milk. If you have any questions or concerns about how to dilute or mix your baby's formula correctly, call your baby's doctor.

Never add undiluted powdered infant formula or concentrated liquid formula directly into your breast milk, and never use your breast milk in place of water to mix concentrated or powdered infant formula.

Ready-to-Feed Formula

In contrast, if you decide to add breast milk into a bottle of ready-to-feed formula, that is OK. This type of formula is not concentrated (i.e., it's already properly diluted), so it does not pose the same concerns as those products that need to be prepared first.

Reasons Not to Mix Formula and Breast Milk

While it is OK to mix your breast milk with an already prepared infant formula in the same bottle, there are some good reasons to offer each one at a time (if possible) that are worth considering as well.

Allowing your baby to finish the breast milk you have on hand before offering supplemental formula means that if your child gets full before finishing the bottle, you'll be tossing formula rather than breast milk. Since breast milk contains more nutritional properties than formula, it is best if your baby gets all of the breast milk that's available. And it's discouraging to have to throw away milk that you worked hard to pump.

A Word From Verywell

Breastfeeding is one example of the best-laid plans not always panning out. Whether you breastfeed, formula feed, or use a combination of the two, remember that making sure your baby is getting adequate nutrition is all that matters. As you find your way, check with your pediatrician if you have any questions.

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4 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Nemours KidsHealth. Breastfeeding FAQs: Solids and supplementing.

  2. Centers for Disease Control and Prevention. Infant formula preparation and storage.

  3. World Health Organization. Guidelines for the safe preparation, storage, and handling of powdered infant formula.

  4. Ballard O, Morrow AL. Human milk composition: Nutrients and bioactive factorsPediatr Clin North Am. 2013;60(1):49–74. doi:10.1016/j.pcl.2012.10.002

Additional Reading
  • Kellams A, Harrel C, Omage S, Gregory C, Rosen-Carole C. ABM clinical protocol #3: supplementary feedings in the healthy term breastfed neonate, revised 2017Breastfeed Med. 2017;12(4):188-198. doi:10.1089/bfm.2017.29038.ajk

  • Lawrence RA, Lawrence RM. Breastfeeding A Guide For The Medical Profession Eighth Edition. Elsevier Health Sciences.

  • Perry SE, Hockenberry MJ, Lowdermilk DL, Wilson D. Maternal Child Nursing Care. Elsevier Health Sciences.

  • Riordan J, Wambach K. Breastfeeding and Human Lactation Fourth Edition. Jones and Bartlett Learning.