How Many Times Can You Safely Reheat Breast Milk?

Breastfeeding and Lactation
Written By Guest User

By Alison Heyerdahl and Vanessa Jupe
Updated Guidelines as of Feb 2025

As a breastfeeding mother, you know how challenging it can be to express and correctly store your breast milk. It takes enormous effort to follow safe storage rules when half asleep after a 3 a.m. feed or when you're running late for a 10 a.m. meeting, and so you really don't want to see that precious liquid gold wasted. What happens if you've heated a bottle of expressed milk that was stored in the fridge and your baby doesn't finish it? The answer varies and will depend on a few factors. Read on to learn more. 

Can You Reheat Breast Milk?

The short answer is yes, it is generally safe to reheat breast milk once; do not reheat your breast milk twice. Reheating can destroy some of the beneficial bacteria and nutrients found in breast milk. Furthermore, it is best to reheat that same breast milk within two hours because bacteria from your baby's mouth could contaminate it. According to the Centers for Disease Control and Prevention (CDC), once you’ve warmed up breast milk (either from frozen or refrigerated) and given it in a bottle to your baby, "if your baby did not finish the bottle, use the leftover milk within 2 hours after the baby is finished feeding. After 2 hours, leftover breast milk should be discarded."

When is it safe to reheat breast milk?

The storage of freshly expressed milk is a different issue from leftover breast milk from a feeding session because bacteria from the baby's mouth enter the milk once she starts feeding. Many mothers reheat excess breast milk that has been saved for longer than two hours (sometimes up to 48 hours) with no problems, but this depends on several factors:

Freshness and Storage: Freshly expressed breast milk has more bacteria-fighting power than previously refrigerated or frozen breast milk. Also, milk that has been frozen for a shorter period will have more antibacterial properties than milk that has been frozen for longer.  

  • Comparison to Formula: Bacteria grow more slowly in any breast milk than it does in formula, so it is generally safer to reheat breast milk than to reheat formula.

  • Hygiene: The cleanliness of your breast milk-expressing equipment, including your breast pump and storage containers, and whether you have washed your hands adequately will also affect the amount of bacteria in the milk.  

  • Baby's Health: If your baby is sick, she may be less able to handle "normal" amounts of bacteria. In this case, you may want to follow stricter guidelines and avoid reheating altogether. 

Can you put breast milk back in the fridge after warming?

It's not recommended to put breast milk back in the refrigerator after warming it. While refrigeration slows the growth of bacteria, rewarming milk multiple times increases the risk of bacterial contamination and nutrient degradation. It's best to discard any leftover milk that your baby doesn't consume within two hours of warming. As Kellymom states: "If you do save the milk for later use, some mothers re-refrigerate the milk and others leave it at room temperature – no studies have been done, so we don’t know whether one method is preferred. We do know that for any food, refrigeration slows bacteria growth. Milk storage guidelines do not include any warnings against rewarming milk, and many moms rewarm with no issues. Many moms smell (or taste) the milk to make sure it still smells fresh."

When shouldn't I reheat breast milk?

There are times when it is inadvisable to reheat breastmilk:

  • Spoilage: Do not reheat breast milk if it has been standing out for too long and you suspect it may be spoiled. You will know if it is off because it will smell sour or have an unusual appearance.

  • Baby's Illness: Do not reheat breast milk if your baby is sick. Breast milk loses some of its antibacterial properties with heating, so it's best to feed your baby a fresh bottle of breast milk that has not been previously heated and cooled down.  

  • Previously Frozen Milk: It's best to avoid reheating previously frozen milk, as it will have reduced antibacterial properties and may spoil more easily. 

How should milk be stored for heating or reheating?

As mentioned previously, storing your milk properly will ensure that you can reheat it with no issues:

  • Hygiene: Before you express your milk into storage containers, make sure you wash your hands and clean all your equipment.

  • Labeling: Always label the container with the date and time the milk was expressed so you know how long you can store it. Breast milk lasts for:

    • 4–6 hours at room temperature.

    • 8 days in the fridge.

    • 6 months in a freezer.

  • Thawing: Always thaw your milk in the refrigerator before heating it up. Avoid thawing at room temperature or in hot water, as this can promote bacterial growth.

How should milk be heated?

Never warm your breast milk in a microwave – it can overheat the milk and can cause degradation of the milk proteins and nutrients.  Warm some water on the stove and place the milk container in the hot (but not boiling) water. 

Avoid Microwaving: Never warm your breast milk in a microwave – it can overheat the milk and create hot spots that could burn your baby's mouth. Microwaving can also cause degradation of milk proteins and nutrients.

  • Safe Warming Methods:

    • Warm Water Bath: Warm some water on the stove and place the milk container in the hot (but not boiling) water.

    • Bottle Warmer: Use a bottle warmer specifically designed for warming breast milk. Follow the manufacturer's instructions.

Remember: Always test the temperature of the warmed milk on your wrist before feeding it to your baby. It should feel lukewarm, not hot.

Disclaimer: This information is intended for general knowledge and informational purposes only, and does not constitute medical advice. It is essential to consult with a qualified healthcare professional for any health concerns or before making any decisions related to your baby's health or nutrition.

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