Your Guide to Mixing Breastmilk and Formula

Your go-to guide for mixing breastmilk and formula.

Your Guide to Mixing Breastmilk and Formula

Hey, mama! Buckle up for your guide on how to mix breastmilk and formula. I am not your all-or-nothing IBCLC, in this space, there are a lot of nuances and I want you to have the breastfeeding journey that is the best fit possible for you and your family. For some families, blending breastmilk and formula aka combo feeding is a part of their story. 

It can be confusing to read through the conflicting advice you find online and settle on something you feel good about. Let’s unpack this topic together and help you figure out what is the best fit for you.

What does it mean to mix breastmilk and formula?

A term you may see is combo feeding, but what exactly does it mean? Combo feeding is feeding your baby both breastmilk and formula. This can happen directly at the breast with a supplemental nursing system (SNS), with breastmilk and/or formula, with a bottle and/or a cup. Supplementing your baby can look a few different ways and there are pros and cons to each method. 

  • Starting with breastmilk, following up with formula as needed to supplement feeds. Your baby may be nursing at the breast or drinking from a bottle. 
  • Supplementing directly at the breast with a Supplemental Nursing System or SNS, you may be supplementing your breastmilk, donor breastmilk, or formula. 
  • What is an SNS? 
  • Supplemental Nursing System: Is comprised of two parts, a holder for the breastmilk or formula and a tube that can be inserted into babies' mouths while they are latched at the breast. Click here to check one out.  
  • Physically mixing breastmilk and formula in the same bottle to feed your baby. 

Safety First: non-negotiables that you want to be sure to do every time

  • Always wash your hands prior to preparing a bottle for your baby. Click here for a guide on bottle introduction. 
  • Always prepare formula, whether ready-to-feed, formula or concentrate to the package directions. Click here for a step-by-step guide to formula preparation.
  • Never use breastmilk in lieu of water for formula preparation. 

Pick the feeding method that works best for your babe and your goals

As with soooo many things with parenting and feeding your baby, there isn't a one size fits all answer to how to combine breastmilk and formula. There is no right and wrong answer beyond making sure your baby is fed and protecting your milk supply.

There are pros and cons to different methods of combo feeding that I am going to unpack below. You may want to try one or all of these examples to figure out what works best for you and your baby.  In Boob School, we offer 2 modules to help with combo feeding. We have an entire module on combo feeding itself and another on how to blend breast and bottle so your baby can easily go back and forth. We also have handouts for breastmilk storage and formula prep. We try to erase the unknowns so you can feel confident feeding your baby however you choose.  

Using breastmilk first, on its' own in a bottle 

  • PROS: Breastmilk has a longer shelf life compared to formula.  Per the Academy of Breastfeeding Medicine, fresh breastmilk can be stored at room temperature for up to 4 hours and in the refrigerator for up to 4 days. Thawed breastmilk can be in the refrigerator for up to 24 hours. Once in a bottle that your baby has drunk from, it can be used for up to two hours. 
  • By giving breast milk separately before formula you have less of a chance of wasting breast milk if your baby doesn't finish the supplement.  With the breastmilk first approach, we ensure the baby is getting all the breastmilk and only offer the formula after they have finished and if they need more. 
  • CONS: It can be frustrating and time-consuming to prepare two bottles for every feed. You have extra bottles to prepare and wash and this can feel overwhelming. You also may still have an unused oz of formula that has to be tossed at the 1-hour mark if the baby drank from the prepared formula bottle.. 

Using breastmilk and formula together in the same bottle. 

  • PROS: It can be quicker and easier to prepare a bottle this way. Your baby may not form as much of a preference for either formula or breast milk.  It helps to blunt the taste of formula, which is much less sweet than breastmilk, and this helps some babies accept the supplement. If you know exactly how much total volume your baby needs, this approach can be super convenient, especially if you have prepared/mixed your formula ahead of time. 
  • For example: if your baby needs a 3-ounce bottle, you can add 1.5 ounces of breastmilk and 1.5 ounces of formula.   According to the book Balancing Breast and Bottle Feeding Your Baby by Amy Peterson IBCLC and Mindy Harmer SLP, since you are offering the breastmilk and formula together some of the enzymes in breastmilk help with the digestion of the formula which can reduce tummy upset in infants. 
  • CONS: You must use the bottle of prepared milk within 1 hour or toss it. Since both breastmilk and formula are mixed in the same bottle we have to go with the “formula rules.” This can end up wasting some of that liquid gold you have worked so hard to pump.

Supplementing directly at the breast with a Supplemental Nursing System (SNS)

  • PROS: You can knock off multiple boxes all in one go. You can have your baby latch at the breast, which helps with nipple stimulation and can help increase supply. You can supplement with breastmilk, your own or donor breastmilk, and/or supplement with formula. It can decrease the overall time you spend feeding your baby as they are able to nurse at the breast and get the additional ounces they need from the SNS.  Baby’s become more efficient at the breast by PRACTICING at the breast so this approach may improve a weak suck and help your baby become a more effective feeder. With this approach, the baby is getting full at the breast which can reduce the risk of the baby developing a bottle preference.  Supplemental nursing systems are especially helpful when a mom has a chronic low supply and will be supplementing the infant long-term. For more on low milk supply check out our blog on low supply, here.
  • CONS: Supplemental Nursing Systems can have a sharp learning curve and can be less intuitive to navigate than a bottle. If your baby is struggling with latching, it may add another layer to getting your baby on the breast, latched correctly, and getting the SNS into their mouth. Working with an IBCLC to help you navigate this new skill is key. 

Starting baby at the breast and topping off with a bottle 

If having your baby nurse at the breast is also one of your feeding goals, combo feeding can be a part of your overall plan in the following ways.

  • You may start at the breast, then move to either a…
  • full formula bottle
  • a combination of pumped breastmilk and formula
  • a full breastmilk bottle
  • or a combination of all these depending on what is available at a given feeding time

Starting baby with a bottle and finishing at the breast

This approach is a favorite of many lactation consultants, myself included. One of the reasons it works well is that the baby is associating getting full with being at the breast rather than the bottle. 

Oftentimes when babies are super hungry they get frantic at the breast latching and unlatching and crying. This can be super stressful for mom and baby! If this is what is happening for you, experiment with offering a small amount in a bottle before latching. The KEY here is going to be HOW we offer the bottle and make sure we are pacing the feeding by holding the bottle horizontally and offering the milk slowly.

Breastfeeding at specific times during the day

You may also save nursing at the breast for specific feeding sessions during the day. Commonly right when your baby wakes up, at nap time, and at bedtime.

This is more likely to be the case for older infants who are taking in calories at mealtimes, in addition to bottles at other times of the day to ensure they are getting enough ounces and calories per day for adequate weight gain. 

  • Example: You breastfeed throughout the day, but offer a bottle of formula in the evening. Important to note: if your goal is to have enough breastmilk to reduce formula supplementation then I would recommend pumping at this time. If you are happy to continue using the formula then keep on truckin'. 

Putting it all together

I hope you walk away from this post knowing that breastfeeding is not an all-or-nothing sum game. It also does not HAVE TO look any specific way. Each parent and baby will have a system and rhythm of their very own, which will likely evolve over the course of your breastfeeding journey. 

It may also look wildly different between your different babies. It is best to stick with what IS working for you and your baby vs. forcing what is not. My goal for you is to walk away from your breastfeeding journey feeling good about the choices you made for yourself and your baby. 

In Boob School, we support all feeding choices and offer help with bottle feeding and supplementation.  We believe breastfeeding is NEVER, “all or nothing,” and have built our program to support families wherever they are. 

Cheering you on, always!



Boob School Founder and CEO

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