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Combination Feeding Breast Milk and Formula Safely

Updated: Sep 15, 2023

Happy family feeding their infant

By: Amy Peterson, LC for Evenflo Feeding

Mixing breast milk and formula safely and how crucial are the volume markings on bottles...

Offering breast milk and formula to a baby is called combination feeding. Babies gain many benefits when they receive breast milk, even when formula is part of the equation. You may have heard the saying, “Some breast milk is always better than no breast milk.” It’s true. Try to provide some of the milk your baby needs via breast milk, even if you also need to use formula. Evenflo Feeding’s Advanced Double Electric Breast Pump is an economical hospital-grade breast pump that can help you maximize your milk production.

Here are some important facts you need to know about combination feeding, beginning with volume markings on bottles, followed by safe formula preparation, and many other details.

When a healthy baby receives only breast milk, we can safely guesstimate how much the baby eats at each feeding. After all, feeding times vary, and the amount of milk a baby takes at each feeding varies. As long as the baby is following a healthy growth curve, it is safe to ballpark quantity, and look at total quantity over a day rather than a specific feeding.

On the other hand, the mixing of formulas requires precision for the nutrients to be accurate. Liquid concentrate and powdered formula options must be mixed with a specific amount of safe drinking water for proper dilution. It is never safe to substitute breast milk for water when reconstituting formula because an over-concentration of nutrients may pose a danger to your baby.

It is important to measure precisely. Measure water with a measuring cup and then pour it into the bottle. Researchers studied volume markings on bottles and learned that 57% of the baby bottles studied had inaccurate or missing markings. Bottle volume markings underestimated or overestimated actual volumes by as much as 43%. For this reason, check the accuracy of the volume markings on the bottles you use for formula preparation, or simply use a measuring cup to measure the water needed to make the formula. Volume markings are not regulated in most countries, so be proactive and measure the accuracy of your preferred bottles and adjust accordingly. (1)

Since powdered formula is not sterile, it is recommended to use water heated to 158°F/70°C for babies younger than three months old or who are medically fragile. 158°F/70°C is the correct temperature to sterilize the powdered formula without destroying important nutrients. (2)

After putting the appropriate amount of heated water into the bottle, then add the powdered formula. Always use a full scoop to measure powdered formula rather than estimating a half scoop. Shake to mix completely. If you plan to use the formula right away, cool it quickly by running the bottle under cold tap water or by placing it in an ice bath.

There are two types of liquid formula—liquid concentrate and ready-to-feed. Liquid formulas are sterile. If you use a liquid concentrate formula, use equal parts of the formula and safe drinking water; it is not necessary to heat the water. Note that if you use a ready-to-feed formula, you do not add water.

To mix breast milk with prepared formula (combination feeding), first put your breast milk in the bottle. At feeding time, pour prepared formula into the bottle of breast milk to equal the approximate amount of milk you anticipate your baby will eat. Since the formula has been prepared precisely in a separate container, the volume markings on the bottle are not that important anymore. You can always add a bit more formula to the bottle at the end of the feeding if your baby seems to want more. Just don’t overfill the bottle with extra formula. You don’t want any of your breast milk to go to waste.

The protein in prepared formula can interfere with some of the anti-infective properties in breast milk. For example, lysozyme activity in breast milk, which protects against bacteria such as E. coli and salmonella, is reduced by 41%-74%. Storing prepared formula separately from breast milk preserves normal lysozyme activity. (3)

Some moms will offer 100% breast milk in a bottle and then offer a separate bottle of 100% formula. If you have enough breast milk to space feedings accordingly, this can be a good option. But if you need to offer breast milk and formula at the same feeding, it’s okay to put both milks in the same bottle as they will mix in the baby’s belly anyway. But do plan to store breast milk and formula separately until the baby is ready to eat, and then mix immediately at feeding time.

Another important reason to store breast milk and prepared formula separately is because they have different time frames for storage. Unused bottles of breast milk can be safely stored in the refrigerator for several days, whereas unused bottles of prepared formula must be discarded within 24 hours.

Once your baby sucks on the bottle, the storage time changes. Breast milk must be discarded within 2 hours, while formula must be discarded within 1 hour. Whenever breast milk is mixed with formula, follow formula storage guidelines.

Cover photo by Anna Shvets


  1. Gribble, K., Berry, N., Kerac, M., and Challinor, M. (2016), "Volume marker inaccuracies: a cross‐sectional survey of infant feeding bottles, Maternal & Child Nutrition," doi: 10.1111/mcn.12388

  2. "The Effect of Nutritional Additives on Anti-Infective Factors in Human Milk", Quan et al, CLIN PEDIATR 1994 33: 325, DOI: 10.1177/000992289403300602

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