Reheating Breastmilk [3 Things You Need To Know!]

You warm up a four-ounce bottle of breastmilk, but your baby only drinks two ounces. Can you reheat the leftover breastmilk or does it have to be tossed? Breastmilk is liquid gold, and we totally understand how upsetting it can be to dump it down the drain!

Reusing and reheating breastmilk is generally not advised. However, there are a few studies showing that it may be safe to an extent, and a few authorities have said it is okay. Let’s break down the risks and safety measures concerning reheating breastmilk.

1. Bacteria in Breastmilk

When you pump, a small amount of bacteria ends up in your milk inside the bottle. Usually, these bacteria are so few, and due to proper storage in the fridge or freezer, they do not have the opportunity to grow.

Additionally, fresh milk can fight bacteria and remain relatively anti-infective. The longer you store the milk, the less bacteria-fighting power it has.

When you give your baby a bottle, bacteria from your baby’s mouth is now present. As soon as your baby begins sucking, the bottle is contaminated.

If these bacteria multiply, they can harm your baby. This is especially true if your baby is premature, suffering from a pre-existing medical condition, or has a weakened immune system. Warm milk is a breeding ground for bacteria.


2. Proper Preparation

The safety of reheating and reusing the breastmilk is dependent on how you prepare and store it. 

You can prepare a bottle with fresh milk, meaning it has been at room temperature for less than four hours. Another option is using milk that has been in the fridge or frozen.

Babies do not require warm milk, yet most babies prefer it. You can give your baby fresh milk at room temperature and similarly, you can use milk directly from the fridge. If your baby prefers a warm bottle, check out these warmers that are quick and safe!

Our Favorites

  • Kiinde Kozii Bottle Warmer- The Kiinde Kozii’s design has your baby’s health in mind. It warms bottles up just a little slower, but only because overheating can destroy nutrients in breastmilk that you really want your baby to get.

  • Munchkins Fast Bottle Warmer- Only one thing needs to be said for the Munchkin Warmer; it heats up a bottle in under 90 seconds. That means minimum time to listen to the hungry cry!

Most moms warm their refrigerated or frozen milk. Only use this milk if it meets proper storage guidelines and is not expired. This means it has been in the refrigerator for fewer than five days and in the freezer for fewer than six months.

To warm the milk, you can either run it under warm to hot water or let it sit in a bowl of warm to hot water. The water should not bring the milk to a boil as this will damage the nutrients and healthy bacteria. For the same reason, you should not heat milk in the microwave or oven.

Once the milk is warm, swirl it to mix in any fat and then test a drop on your rest for temperature. As long as the temperature feels fine, the milk is now ready for your baby.


3. What To Do with Leftover Breastmilk

You fed your baby but you still have some milk leftover and you hate to just throw it away. Is it safe to reheat or reuse later? This is where things get tricky.

Recommendations against Reheating Breastmilk

Most pediatricians and organizations advise against reheating or reusing breastmilk. Once the milk has been warmed and given to a baby, the risk for bacterial contamination is high. The CDC advises, “if your baby did not finish the bottle, the leftover breast milk can still be used within 2 hours after the baby is finished feeding. After 2 hours, leftover breast milk should be discarded”[1].

Similarly, Today’s Parent states, “It’s not safe to reheat a bottle of breastmilk”[2]. The thought behind these statements is reheating and reusing the milk it is not worth the risk to your baby’s health. You should dispose of all remaining milk within a two-hour period. 

Other authorities further specify the situations in which you should not reheat milk. If your baby is sick, their immune system is suppressed. They are more susceptible to bad bacteria from milk with any contaminants. If the milk has been previously frozen you should not reheat it (or refreeze it). This is because the freezing process weakens the milk’s bacteria-fighting properties, resulting in an increased bad bacteria count.

Finally, if milk is clearly showing signs of spoilage, you should not reheat it or reuse it. Overall, this approach errs on the side of caution. If your baby has not finished the bottle, it is best to dump the remaining milk along with the contaminating bacteria down the drain.

Recommendations for Reheating Breastmilk

The lack of a sufficient amount of data regarding the reheating and reusing of breastmilk leads others to believe that it may be safe to offer previously warmed milk. It is important to note that these statements are in support of reusing milk but largely ignore or advise against reheating milk.

This is in part because of the time frame in which the previously fed milk should be used. Throw the bottle away if it has been out for 2 hours. 

At room temperature, the bottle should not cool significantly within the two-hour window. Therefore, you should not need to reheat the milk. But is it still safe to use?

Expert Opinions About Reheating Breastmilk

Jan Barger, R.N., M.A., IBCLC states “it would seem to me that expressed breast milk, fed to the baby, partially consumed, and then refrigerated, could be used for one more feeding no more than four hours later. That’s probably fairly conservative,”[3] concluding that bacterial contamination will cause the milk to spoil, but there is no way of knowing how long this will take.

The Academy of Breastfeeding Medicine notes, “Once a baby begins drinking expressed human milk, some bacterial contamination occurs in the milk from the baby’s mouth. The duration of time the milk can be kept at room temperature once the baby has partially fed from the cup or bottle would theoretically depend on the initial bacterial load in the milk, how long the milk has been thawed, and the ambient temperature. There have been no studies done to provide recommendations in this regard. Based on related evidence thus far, it seems reasonable to discard the remaining milk within 1”“2 hours after the baby is finished feeding”[3].


The common thread here is that too few studies have been done to conclude officially whether reheating and reusing previously used milk is safe or not. It would appear that as long as the milk was not frozen, your baby is healthy, and you use it within a one to two-hour period, the milk should be safe.


What Should You Do About Reheating Breastmilk?

In the end, what you do with your leftover breastmilk is up to you. There are advocates of, and studies upholding both opinions. If you are worried for any reason that your breastmilk may be harmful to your baby, it is best to toss it.







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